The invention discloses a detecting device of a circuit breaker. The detecting device comprises a bottom plate, a cushion block, a main gate switch, a detecting mechanism and a sliding mechanism. Thesliding mechanism consists of four horizontal grooves and two vertical grooves; the four horizontal grooves are respectively installed at two inner fixed blocks and two outer fixed blocks one by one and straight lines of the four horizontal grooves in length directions are parallel to each other; and the two vertical grooves are arranged at the two ends of the cushion blocks and are installed on the bottom plate, wherein the straight lines of the vertical grooves in length directions are parallel to a telescopic rod of a cylinder. Short cylinder cooperating with the vertical grooves are arranged at the horizontal grooves, so that the inner fixed blocks and the outer fixed blocks can slide along the vertical grooves when the short cylinders slide along the vertical grooves. The detecting device is capable of screening circuit breakers preliminarily to reduce the number of circuit breakers with faults in follow-up detection, so that the time is saved.