Abstract: Abstract: Hydraulic shock often happens when valves are opened or closed suddenly, and it may cause a series of trouble for the hydraulic system. If the motor in the hydraulic system has a huge initial load, hydraulic shocks might be larger when the valve is closed. When a hydraulic motor drives a plate whose rotary inertia is as large as 17 kg?m2 running at a highest rotating speed of 400 r/min, if the control valve is suddenly closed, the hydraulic shock is not ignorable.In order to determine the fatigue damage mechanism for a specific five-star radial piston motor under the hydraulic shock, the AMESim software was employed to study hydraulic shock waves and its highest pressure value. The simulation model was built under the guide of the hydraulic schematic of the hydraulic system. In the model, relief valve 1's opening pressure was set to be 35 MPa, while relief valve 2 and relief valve 3 were both set to be 31 MPa, relief valve 4 was 21 MPa. An IR-C-IR submodel was employed in hose 7 and 8 to simulate hydraulic waves in hoses, while the other pipes were considered as rigid pipe. The control valve was open at the very beginning of the simulation, and closed 4 seconds later, until 16 seconds later the simulation terminated. Simulation results showed that a hydraulic shock whose pressure as high as 36 MPa occurred just after the control valve was shut down, and the amplitude of the wave decreased step by step. After that, the highest pressure value was considered as the pressure boundary on the simulation of ANSYS CFX to simulate impact pressure acting against on valve plate, and the results showed that the impact pressure could be up to 34.9 MPa. This high hydraulic pressure shock pushed the valve plate to knock the upper shell hardly, and the frictional moment between valve plate and upper shell could reach 60.02 Nm. This moment was taken as the load into pintle valve to calculate the fatigue life of the pintle valve, and the results showed that the fatigue life only had 2 197.6 times at the contact area. It was prone to fatigue damage on this area.In the experimental section, the motor started and stopped frequently under the control of the valve. And during this cycle, loud noises came from the motor. After several times of cycle, the motor stopped running, and could not rotate any longer no matter how the valve shifted. Visible scratches in the damaged motor indicated that the valve plate and the upper shell had mutual friction violently. And the pintle valve was badly damaged in the role of hydraulic shocks.After all, several conclusions can be drawn that: 1) The hydraulic shock could be very harmful to motor, and 2) Methods taken in the paper can be used to forecast the fatigue damage of motor caused by hydraulic shocks.