1. Introduction PLCs, due to their powerful functions, simple programming, and convenient maintenance, especially their high reliability and strong adaptability to harsh industrial environments, have been widely used in the water supply industry. However, due to harsh on-site environmental conditions, high humidity, and various industrial electromagnetic and radiation interferences, the normal operation of the system can be affected. Therefore, it is essential to pay attention to the anti-interference design of the project. The interference sources of PLCs in water plant applications mainly fall into three categories: interference introduced by the power supply system, interference introduced by the grounding system, and interference introduced by the input and output circuits. If the interference problem of PLCs is not properly solved, the system will not be able to operate reliably, which will affect the normal water supply. Therefore, it is necessary to discuss the interference problem in PLC application systems. This paper mainly discusses three anti-interference techniques for PLCs. 2. Analysis of Anti-interference Technical Countermeasures To prevent interference, hardware and software anti-interference measures can be adopted. Among them, hardware anti-interference is the most basic and important anti-interference measure. Generally, it starts from both resistance and prevention to suppress and eliminate interference sources, cut off the coupling channel of interference to the system, and reduce the system's sensitivity to interference signals. 2.1 Interference introduced by the power supply system Interference from the power grid and frequency fluctuations will directly affect the reliability and stability of the PLC system. How to suppress the interference of the power supply system is the main link to improve the anti-interference performance of the PLC. (1) Install a filtering, isolation, shielding, and switching regulated power supply system. The purpose of setting up a filter is to suppress the transmission of interference signals from the power line to the system. When using an isolation transformer, it is necessary to pay attention to the following: the shielding layer must be well grounded; the secondary connection line should use double winding (to reduce interference between wires). The primary winding and secondary winding of the isolation transformer should be shielded separately. The shielding layer of the primary winding is connected to the neutral line of the AC power grid; the shielding layer of the secondary winding and the shielding layer between the primary winding and the primary winding are connected to the DC terminal. In order to suppress the voltage fluctuations of the power grid caused by the start-up and shutdown of large-capacity equipment (such as water pumps) and maintain the voltage stability of the power supply, a switching regulated power supply can be used. (2) Separate power supply system The PLC controller and I/O system are powered by their respective isolation transformers and are separated from the main power supply. In this way, when the input and output power supply is interrupted, it will not affect the power supply of the controller. 2.2 Suppressing Interference Introduced by Grounding System PLC systems are divided into logic circuit grounding and power circuit grounding, with three methods: common ground, floating ground, and chassis common ground and circuit floating ground. Generally, it is best to use separate grounding for the controller and other equipment. When grounding, pay attention to the following: the grounding wire should be as thick as possible, generally greater than 2mm²; the grounding point should be as close to the controller as possible, with a distance of no more than 50m between the grounding point and the controller; the grounding wire should avoid high-voltage circuits and main circuit wires as much as possible. If avoidance is not possible, they should intersect perpendicularly, and the length of parallel lines should be minimized. Practice has proven that grounding is often an important means of suppressing noise and preventing interference. A good grounding method can greatly suppress the coupling of internal noise, prevent the intrusion of external interference, and improve the system's anti-interference capability. 2.3 Suppressing Interference Introduced by Input/Output Circuits To achieve complete isolation of input/output circuits, optocouplers have been widely used in control systems in recent years, becoming one of the most effective measures to prevent interference. Optocouplers have the following characteristics: First, because they are sealed within a casing, they are not affected by external light interference; second, because they transmit signals via light, the ground connection between various component circuits is severed; third, the dynamic resistance of the LED is very small, while the internal resistance of interference sources is generally large, thus the interference signal transmitted to the input and output of the optocoupler becomes very small; fourth, the transfer ratio of an optocoupler is generally much smaller than the amplification factor of a transistor, making it far less sensitive to interference signals than a transistor, and the LED in an optocoupler only emits light when a certain current flows through it. Therefore, even under conditions of high interference voltage amplitude, the LED cannot emit light due to insufficient energy, thus effectively suppressing the interference signal. Since the linear region of an optocoupler is generally limited to a specific range, it is essential to ensure that the range of variation of the transmitted signal remains within the linear region. To ensure linear coupling, both strict selection of the optocoupler and appropriate nonlinear correction measures must be implemented; otherwise, significant errors will occur. 3 Conclusion The anti-interference design of PLC control system is a complex system engineering project, involving specific input and output devices and industrial site environment. When designing an anti-interference system, it is required to comprehensively consider various factors.