The mathematic model on the leaking flow field of the piston sector considering the high press and high velocity in the piston pump was established, and the numerical simulation and analysis were done. At first, the 3-D dynamic analysis on the piston sector was done and the force change principle of the piston sector was analyzed in the Matlab program. Then, the theoretical mathematic model about the flow field in the piston sector with the acentric and inclined armillary seam was built based on two parameters: the inclined angle φ and the acentric distance e. These two parameters φand e were numerically solved with the theory of dynamic pressure supporting and the result of the dynamic analysis on the piston sector. The numerical analysis on the leaking flow field in the piston sector with the acentric and inclined armillary seam was done in the Matlab program with the method of 1-D searching in some conditions. At last, the simulation was done with Fluent and the experimentation was conducted. The computing and analysis results indicate that the armillary seam in the piston sector is acentric and inclined considering the high press and high velocity, the change extent of the leaking flow rate in the piston sector is broader than which in the piston sector with the assumed homocentric armillary seam, but the average leaking flow rate is less in practice.