Abstract: The paper studies the effect of plunger diameter ratio on the pressure peak of pulse pressure wave. The test chooses four group plungers with difference diameters, collectes and analyzes the pressure transient state data from the outlet of the devices and the velocity data of four plungers. And the results determine the plunger structure range of the maximum pressure peak, and the plunger diameter ratio is between1/3 and 1/2.