In order to improve the weighing accuracy and stability of granule packing machine, a granule packing machine weighing system was designed based on PLC. The structure and technological process of granule packing machine were analyzed, and the control system structure was given, including core processor PLC, AC controller, servo motor driver, sensor, three-phase motor and servo motor, etc. Taking weight control as the main research object, a particle swarm fuzzy PID weighing controller was proposed to improve the convergence speed, universality and portability of the weighing control system. Finally, the simulation and experimental research were carried out. The relevant results show that, compared with the fuzzy PID controller, the system has a faster response speed and a shorter time to reach the stable state after particle swarm optimization algorithm is added. The actual packing error is only 0.528%. The weighing control system can effectively improve the weighing accuracy and is conducive to improving the automation level of the granule packing machine.