Abstract: This article mainly describes the interlock protection function design in the cyclotron control system of HIMM (Heavy Ion Medical Machine). Aiming at the interlock requirements of ion source, radio frequency, vacuum, power supply, cooling water and other subsystems, we designed an interlock system based on device+PLC+software structure. This design ensures the normal work of interlock function in case of power down, line connection failure or abnormal data transmission. The reaction time reached milliseconds for hardware interlock and sub-second for software interlock. The whole function was verified in the process of cyclotron testing and ensured the beam tuning of the whole accelerator.