Abstract: A medical capsule micro robot model with radial clearance compensation function driven by external rotating magnetic field is proposed.The magnetic drive characteristics are studied and the mathematical model of magnetic torque is established.Based on the resistance torque characteristics of robot in liquid,the non-synchronized rotational speed phe- nomenon caused by magnetic drive torque to overcome liquid resistance torque is studied,and the relationship between non-synchronized rotational speed and screw parameters is analyzed.Swimming experiments show that non-synchronized rotational speed phenomenon doesn't happen in the robot magnetic driving system,and propulsion and resistance torque of the capsule robot can be controlled with radial clearance adjustment.