Abstract: This paper describes the development and application of the closed-loop control system of hydraulic injection molding machine. The system realizes the closed-loop control of the electro-hydraulic proportional valve of the injection molding machine and develops the upper computer monitoring software to realize the online adjustment of PID parameters. Keywords: PID control, monitoring software, microcontroller, injection molding machine 1 Introduction As one of the leading products of plastic machinery, it is very necessary to study its closed-loop control. Although the closed-loop control of injection molding machine is not complicated, it is very difficult. The main factors affecting the surface quality and weight repeatability of injection molded products are: injection speed, holding pressure, back pressure, and the endpoint position of injection and melting [1]. The injection process is divided into three main stages: injection, holding pressure, and melting. The above three parameters can be closed-loop controlled in each stage. The states of the above parameters will be different for different machine models or different environments (temperature, humidity, etc.) of the same machine model, different molds, different injection process parameters, and even different machines of the same model. The injection molding industry today demands rapid production readiness, which open-loop control clearly cannot meet. Meanwhile, closed-loop control parameter adjustment should be as simple as possible. To achieve optimal control, it is essential to understand the parameter information throughout the entire process, and the control parameters should be adjusted accordingly to environmental changes. To realize closed-loop control of the injection molding machine's hydraulic system, and to ensure its simplicity and ease of implementation while meeting industrial production requirements, the development of a closed-loop control system with adjustable parameters is imperative. Therefore, a closed-loop control card was developed as the carrier of the system control software, along with corresponding upper-level monitoring software. The combination of these two technologies enables closed-loop control of the injection molding machine's hydraulic system. For details, please click: Application Research of Closed-Loop Control System for Hydraulic Injection Molding Machines.