<p>The self-priming characteristic of the vortex pump is one of its most prominent and competitive advantages. At present, research on vortex pumps mainly focuses on energy characteristics, design optimization and other aspects, while there are not many studies on the self-priming characteristics of vortex pumps. This study conducted an in-depth investigation on the self-priming process of vortex pumps by using numerical calculation methods. It analyzed the interaction mechanism between the gas and water phases during the self-priming and exhaust process of vortex pumps and obtained the operating laws of the gas in the impeller and volute during the self-priming process. The results show that the two-phase mixture of gas and water mainly occurs in the interface area between the volute inlet and the water absorption chamber, and the liquid phase is encapsulated by the gas phase when it flows out from the root and top of the blade. There is a gas–liquid stratification phenomenon on the volute wall, which is not conducive to the discharge of gas. There is a large-scale vortex structure in the gas–liquid separation chamber, which causes gas mass retention and delays the exhaust process. During the exhaust process, the axial force of the impeller first decreases and then increases with the change of air content. Throughout the entire process, the impeller is constantly subjected to a radial force directed towards the fourth quadrant.</p>