High pressure water-hammer is equipment which can produce pressure impact.It stores energy and releases stored energy in a twinkling by spring,can excite pressure impact in fluid pipeline.According to analysis on the process of motion of the water-hammer,its energy equation was established and simplified reasonably,a computation formula of its peak impact pressure was suggested.The influencing factors of peak impact pressure were analyzed based on experimental data,and it was proved that the frictional resistance of piston seal is the most important influencing factor for the peak impact pressure.It provides a theoretical support for pulsed water injecting into coal seam.