Abstract: In order to solve the difficult problem of low permeability coal seam water injection, this paper researched the peak pressure of water hammer. This paper firstly introduced the mechanism of pulse pressure wave produced by plunger impact, established the differential equation that described the plunger impact process, finally got the mechanism that the plunger 's diameter impacted the pressure peak of pulse pressure wave. Then based on laboratory tests, the plunger's diameter impacting on the pressure peak of pulse pressure wave was discussed. The study would be good for improving the device structure and increasing the pressure peak, and then would improve the effect of preventing dust by water-injecting and gas drainage in the coal seam that the permeability was low.