The invention discloses a speed changer, comprising a motor, an output shaft, a gear transmission mechanism and a control system, wherein the motor and the output shaft are connected by the gear transmission mechanism; the control system comprises a control unit and a driving device, the driving device is provided with a moving piece which can move between a first position and a second position inthe gear transmission mechanism, and when the moving piece is in the first position and the second position, the output shaft has different output characteristics. When load torque changes to a preset value, the control unit firstly adjusts the torque transferred by the motor to the gear transmission mechanism, and then controls the driving device to realize automatic gear shift. The speed changer of the invention can solve the problem of gear beating in the process of automatic gear shift, the electrical properties are improved, and the service life is also extended.