According to the study on the failure mechanism of gear pump face seal, the key factor of face seal failure is the end plate’s overturning moment induced by the asymmetric pressure distribution in a master slave gear cavity. For a type of high pressure gear pump, the nonlinear differential equation of gear pump pressure transition zone was established to analysis the internal flow field of the master slave gear cavity. Then the test system of gear pump cavity pressure was built to verify the failure mechanism of the floating end plate of gear pump. Test data shows that the error between experiment and theoretical value is less than 5.0%. The floating end plate’s overturning moment about x axis is 56.51 N·m under rated conditions.