In order to clarify the abnormal failure mechanism for the spring of the water injection pump, the velocity and the pressure are simulated for hydraulic unit in different driven angle. The eccentric load is found in the ansys, which is the main factor for leading to the exceptional failure of the spring. The improvement method is presented by using Computational Fluid Dynamics (CFD) technology. Flow field of the improved hydraulic unit is more uniform after the numerical simulation. The effect of eccentric load is greatly reduced for the suction spring. At the same time without changing the rate of flow, section surface of the suction valve and the discharge valve is appropriately reduced to bring down the load of the spring. The fluid dynamics analysis and optimization of the hydraulic unit has been completed in this paper. This study has great significance in perfecting the working theory of the water injection pump and raising the robustness of the pump’s motion.