Abstract: Gantry is an important part of proton radiotherapy equipment. Its main features are its large mass，

large size and large moment of inertia. It is necessary to ensure the smooth acceleration and deceleration and precise positioning of the gantry. In response to the positioning control requirements of the gantry，the hardware and software parts of the control system were selected and designed，the control model of the system was studied，the parameters were adjusted，and a PLC-based proton therapy gantry positioning control system was developed. Field test results show that the developed proton therapy gantry positioning control system can ensure that the gantry has an angular positioning accuracy of 0.011 3°under the conditions of an angle range of -180°～180°and a rotation speed of 1 r/min，which satisfies the work requirements for proton radiotherapy equipment.

      Key words: proton radiotherapy equipment；large inertia load；gantry；positioning control