Abstract: In view of problems of complex structure, poor operability and low control precision of existing bolt pretension torque control device of coal mine, a mechanical bolt pretension torque closed-loop control device was designed. The device measures torque of gear ring by change of liquid pressure of the liquid-filled bellows and acts the pressure on slide valve. When the torque reaches control value, the slide valve can be action and makes the bolt pretension circuit cut off, so as to complete a bolt pretension process. The simulation and underground test results show that the device can effectively control torque within 480-510 N·m with fast response，high reliability and control accuracy of more than 90%. Temperature and angle have no influence on the measurement results. The torque multiplier's transmission efficiency and the friction factor of friction pad lead to the control result error of 6.2% and 10% respectively. The friction pad with smaller friction factor should be used and the torque multiplier should be maintained regularly.