A motor driving circuit is applied to a motor unit, a pushrod unit, and a load unit. The motor unit is driven by the motor driving circuit. The pushrod unit is driven by the motor unit to lengthen or shorten. The load unit is pushed by the pushrod unit. A relay unit of the motor driving circuit is provided to brake the motor unit, thus raising the self-locking force of the motor unit when the pushrod unit lengthens to the maximum length or shortens to the minimum length, or the power supply is cut off.