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Taking low speed high torque water hydraulic motor as the main research object, the movement of port plate, rotor and piston were simulated by the dynamic mesh technique and UDF (user defined function) in Fluent．And 3-D internal flow field of water hydraulic motor and the model with damping groove were computed numerically. Based on the minimum through-flow area equation and the formula of pressure change, the mathematical model of the pressure of plunger cavity was established at various percolation angles. The research showed that the original model of plunger cavity had significant pressure overshoot, pressure oscillation, pressure surge, and velocity jump. When the model was modified, the internal flow field of plunger cavity became more stable, so that the shock in valve port process could be effectively reduced and the phenomenon of pressure rise was reduced and the oscillation amplitude and time were decreased. The aim of pre-loading were well achieved. It was also found that the larger the percolation angle of damping groove was, the more effective the pre-loading was. The research could be an important reference for port plate optimal design.