In order to study the gas-liquid two-phase flow during the gas-liquid mixing and separating process, a flow-ejecting self-priming pump named JETST-100 was chosed. Based on Eulerian-Eulerian multiphase flow model, the transient numerical simulation of the gas-liquid mixing and separating phenomena on the pump chamber was carried out using CFX software. The distributions of pressure, velocity and gas-liquid phases field inside the pump were obtained, and the change of the air volume fraction by monitoring the points on the gas-liquid separation chamber outlet and jet aerator inlet were analyzed . The test and simulation results show that the flow of liquid from the guide vane formed a large amount of velocity circulation. And the water of the pump chamber reduced with the large amount of water into the outlet channel. It was found that adding the reinforcing plate could prevent the generation of circulation on guide vane back, and improve the self-priming performance of the pump.