The transient self-priming process is characterized by a complex gas-liquid two-phase flow in the pump, and the gas-liquid mixture and separation effects play a crucial role in gas exhaustion, which further affect the self-priming performance. The collaboration system between the external characteristic of self-priming performance and gas-liquid two-phase flow characteristic during self-priming process was built. Then the experimental instant pump inlet and outlet pressure, pump inlet and outlet flow rate, rotational speed were obtained and programmed by the LabVIEW software. Gas-liquid two-phase flow phenomena in the impeller, volute and gas-liquid separation chamber was observed by high-speed photography system. Results showed that the self-priming process was stable under dynamic conditions before liquid flow was increased. The stable gas-liquid separation interface which appeared at the outlet of the diffuser contributed to gas-liquid separation and gas exhaustion. Bubbles which convoluted at the outlet of the diffuser were not conducive to gas exhaustion in time. During the start-up process, the gas-liquid velocity and the number of bubbles at each section of the volute were in the oscillatory process due to the unstable pressure differential in the impeller. At last instant self-priming process, there was a sudden change in flow rate and pressure. Drops in the gas-liquid separation chamber and bubbles in the diffuser were increased sharply, and then decreased rapidly. The results testified the transient effect during the last instant self-priming process. Microscopic variations of gas-liquid two-phase flow are consistent with macroscopic variations of pressure, flow rate and rotational speed measured by experimental instruments.