The invention provides a double-speed output planetary gear speed reducer and relates to a speed reducer. The speed reducer comprises a machine base part, a stator part I, a stator part II, a rotor part I, a rotor part II, a controller I, a controller II, a rotating shaft bracket part and a dual planetary gear speed reducing part. In the operation process of the speed reducer, the output rotatingspeed of an output shaft I is related to the difference between rotating speeds of two motors; the two motors can be in high-speed operation state all the time; the rotating direction of the output shaft I of the speed reducer can be changed under the condition that rotating directions of the two motors are not changed, so that a load device is driven to achieve the functions of frequently accelerating and decelerating, frequently starting and stopping and even frequently changing the rotating direction of the load device; the speed reducer is capable of ensuring that the motors operate underthe high-efficiency operation state; meanwhile, the response speed can be improved when the rotating direction of the load device is changed by the speed reducer.