The invention discloses a sensor for measuring relative rotating speed of two concentric rotating shafts of a double-rotor motor, belongs to the field of speed measuring sensors, and aims to solve the problem that the prior sensor cannot directly measure the rotating speed difference of a double-rotor structure of a motor system. The sensor consists of a reluctance type rotary transformer, a signal transmission device and a signal processor; the reluctance type rotary transformer consists of an outer rotary change rotor, an inner rotary change rotor, an exciting winding, a sine winding and a cosine winding; one of the outer rotary change rotor and the inner rotary change rotor is provided with even teeth, the other rotor has a winding-free laminated salient pole structure, and each tooth is intertwined with the exciting winding, the sine winding and the cosine winding in the same distribution; two ends of winds of the exciting winding, the sine winding and the cosine winding are connected with the signal transmission device respectively; and the signal transmission device is connected with the signal processor. The sensor is used for measuring the relative rotating speed of the two concentric rotating shafts of the double-rotor motor.