The high-speed photography system was built to investigate the self-priming process of external mixing self-priming pump to analyze the influence of gap between impeller and tongue on self-priming performance of pumps, and then further grasp the self-priming mechanism. The digital images processing technique based on Matlab was applied to compute the number, average diameter and motions of bubbles in the area of diffuser. The tongue gap was resized and the self-priming time was measured to research the internal relations between the bubbles characteristics and self-priming mechanism. The results show that self-priming process is a complicated unsteady gas-liquid two phase flow and always accompanied by generation, collapse, gathering and splitting of bubbles. During the self-priming process, bubbles in the left area of diffuser are smaller and denser, which are easier to be discharged out of the pump. On the contrary, bubbles in the right area vibrate and rotate, which goes against self-priming and wastes energy. When the gap is increased from 1.0mm to 2.5mm, the average diameter of bubbles in the diffuser is increased from 1.2mm to 1.9mm and the self-priming time is increased from 28s to 113s. It means that the bigger the gap is, the bigger diameter of bubbles from tongue to diffuser will be and the longer the self-priming time will be.