Abstract: The self-priming process of self-priming pump was a very complicated gas-liquid mixed flow and separation process.In this article,a unsteady numerical simulation was applied to simulated the self-priming process of self-priming pump,with the initial condition was that the inlet and outlet pipe filled with air,and then self-priming time was estimated;The simulation results showed that gas inhaled into and discharged of the pump were mainly concentrated in the early stage of the self-priming process,the air void fraction reached up to 62.4% and 45.3%,respectively at the impeller inlet and the pump outlet in the initial stage;The change of air void fraction in the impeller inlet was more drastic than that of the pump outlet;In addition,self-priming performance was affected by the gap between impeller and guide vane,and the data showed a negative correlation between gap and self-priming time,which revealed the lager gap was beneficial to self-priming of the pump.