Abstract: HIMM (Heavy Ion Medical Machine) is a self-designed and independently developed medical accelerator device in China, the stability and reliability of the whole accelerator control system is higher than that of the industrial system. Ion source gas valve is an important component of ion source to generate beam. The gas flow rate and mixing ratio directly affects the vacuum, ionization efficiency and stability of ion source. This paper introduces the gas control system based on Mitsubishi servo motor and Phoenix PLC for HIMM ion source system. The system structure, hardware composition, PLC program, interface program and data analysis are presented. In order to ensure the stability of the system, hard interlocking with vacuum values and remote switching functions are added to the motion control. The actual test shows that the system runs stably and the positioning accuracy can reach 0.1?, which meets the design requirements.