This paper proposes a power conversion system for a treadmill with auto-transferring modes between a motor and a generator. With the grid-connection function, the kinetic energy of the treadmill can be converted into electric power and fed into the grid. An infrared sensor is equipped in the control loop to detect the position of the user on the treadmill. Based on the position of the user, the reaction torque of the treadmill is controlled by varying the output current of the generator-mode or transferring operation into the motor-mode to keep the user moving within the defined operating area of the treadmill. The bi-directional power conversion system developed for the treadmill includes a three-phase AC-DC converter and a single-phase DC-AC converter connected to a grid. In addition, the transformation of abc-dq0 is employed for better torque control via current amplitude control. The developed power conversion system has been implemented in a newly designed treadmill to verify the feasibility and effectiveness of the proposed auto-mode transferring control strategies of the generator and motor.