The invention relates to a high-frequency cavity constant-temperature device, a control method and a proton/heavy ion accelerator. The constant-temperature device comprises a cooling device and a constant-temperature heating device, wherein a bypass device of the constant-temperature heating device is in bypass connection with a delivery pump of the cooling device and a cooling liquid inlet end ofa high-frequency cavity, a heater of the constant-temperature heating device is located outside the bypass device and an input end cooling loop and between a cooling liquid outlet end of the high-frequency cavity and the delivery pump, the constant-temperature heating device shares the delivery pump of the cooling device and a cooling pipeline in the high-frequency cavity and can heat and maintain the standby temperature of the high-frequency cavity in a shutdown vacuum breaking state, more specifically, an input end of the delivery pump is connected with a backflow cooling control valve so as to adjust and control the fluid delivery amount of the delivery pump. The constant-temperature device is advantaged in that the exercise time of the high-frequency cavity is shortened, and an accelerator is put into operation more quickly.