The invention discloses a large motor belt tensioning device comprising a device body, a motor base and a baseplate base. A nut base is arranged in the middle of the right lower hinge base. The nut base is fixedly connected with the lower hinge base. A rotating shaft is arranged in the middle of the nut base. The rotating shaft is flexibly connected with the nut base. The right end of the rotatingshaft is provided with a lead screw segment which is fixedly connected with the rotating shaft. When the belt of the motor is adjusted by using the device body, the servo motor is turned on to drivethe rotating shaft to rotate and the lead screw segment of the tail end of the rotating shaft is enabled to screw in or screw out of the nut base to drive the connecting rod to move leftward or rightward. In the movement process of the connecting rod, the upper hinge base is also driven to move to be staggered from the position of the lower hinge base through the fixed sleeve ring of the tail endso as to complete adjustment of the belt tensioning degree of the motor. Adjustment of the belt tensioning degree can be completed without performing direct manual action on the motor in the adjustment process so that use of the device body is enabled to be more convenient and rapid and the device has great development prospect in the future.