A common vertical self-priming centrifugal pump was selected as the research object. By using unsteady numerical simulation, the transient gas-liquid two-phase flow in the self-priming centrifugal pump during the self-priming process is explored. The main innovation in the simulation is that a section of the suction pipe filled with air is chosen as the initial condition, consisting with the actual self-priming conditions. The gas-liquid two-phase distribution as well as the pressure and velocity in relation to the time are computed and analyzed. The flow rates of both phases with time at the pump inlet and outlet are obtained based on the simulation, which could be used to estimate the self-priming time and other performance parameters. It is found that most of the gas are inhaled into and exhausted out of the pump during the early stage of the self-priming process. The air void fraction could reach at 30.9% and 20.2%, respectively at the impeller inlet and the pump outlet at some moments. The hole on the volute plays a significant role for gas exhaustion during the self-priming process. The air flow rate of the orifice accounts for 20%-25% of the entire air flow rate. The simulation method and results proposed have certain theoretical and engineering values for analyzing self-priming process and performance of self-priming pump.