Aiming at high requirement of the fit in the hydraulic tappet stem test, and the difficultly for inspection the gap of hydraulic tappet stem, bring forward a method of real-time measurement. Adopt grating for measuring the displacement of subsidence, PLC for acquisition the pulses which the grating output. The upper computer exchanges the data through serial communication with PLC, and handles the acquisition data. Set the process’s priority for ensuring the real-time measurement. The result indicates that the method ensures the real time of measurement, and it can provide the reference for inspection of the gap.