Abstract: In order to determine the failure mechanism of the unloading valve’s main valve of the emulsion pump, according to the actual working conditions of the unloading valve, the hydraulic model was established. During analysis the spool’s speed and displacement and valve’s pressure and flow rate in the opening and closing process of the main valve, it was found that the main valve spool vibrated severely during the opening process, and the pressure and flow fluctuation range was larger compared with the closing process, it had a greater impact on the failure of the main valve. After theoretical calculations, the steady-state opening amount of the main valve is obtained, the steady-state and transient opening process of the main valve are analyzed, and the changes of the main valve pressure, velocity, turbulent kinetic energy and fluid trajectory under various states are obtained, the damage reason at a/b/c of main valve was analyzed. The results show that a’s injure of the valve core is mainly caused by transient cavitation and vortex, b’s injure is mainly caused by transient and steady fluid vortex, and c’s injure of the valve sleeve are mainly caused by transient and steady cavitation.