A type of diffuser which had two asymmetrical exits and a separating flow board was placed at the inlet of the impeller of flow-ejecting self-priming centrifugal pump. The gap between the separating flow board and the impeller was 1.0mm. Based on Mixture multiphase flow model, the standard k—ε turbulence model and SIMPLE algorithm, unsteady numerical simulation of the self-priming process was carried out by using Fluent software. The distributions of pressure, velocity and gas-liquid phases field inside the pump, and also the changing regularity of pressure, gas phase volume flow rate and liquid mass flow rate on the two exits of the diffuser with the change of timestep were obtained. The results show that the pressure in the diffuser gradually increases along the direction of flow and appears larger change gradient, the pressure inside the each runner of the impeller approximate symmetric distribution.The flow parameters appear periodic change on the two asymmetrical exits of the diffuser. The gap between the separating flow board and impeller have not only prevented the formation of circulation of the fluid, reducing hydraulic loss and improving the efficiency of the pump, but also quicken the mixing of the gas-liquid near the gap, shortening the self-priming time of the pump.