In order to change the traditional method in detecting over-speed governor, in which the structures of measuring tools are complex, the testing accuracy is low, and the workload is heavy. A device with simple structure, easy operation, and high precision was designed. Inspectors need not measure the wheel diameter of over-speed governor, they simply measure the wire rope diameter and the slot depth. In the process of the wheel rotating accelerated, the mechanical device will issue two pulse signals when the speed reaches to electrical speed and mechanical speed. Detecting device calculates the pitch speed in the two moments based on a formula: electrical speed and mechanical speed. So inspectors can determine whether the over-speed governor is qualified based on the two tested figures.