At present, there is still a phenomenon of manual bag weighing on the modified plastic production line. In order to meet the needs of automatic loading into bags and online weighing, a set of highprecision online weighing system was designed. The system uses C8051F021 singlechip microcomputer as the main control chip of the system. The voltage signal in the material filling process is collected by two pressure sensors. Through the masterslave structure, the obtained data is transmitted to the upper computer through the serial communication interface. The PC software obtains the weight information after calibration of the sensors, and can control the charging mechanism through the host computer to achieve precise control of the charging process and improve production efficiency.