Rotational flow self-priming pump’s volute is different from conventional pump, with special flow structure. 3-D unsteady turbulent flow of rotational flow self-priming pump under design condition was simulated by using large eddy simulation method and sliding mesh technique. In the impeller and volute, pressure distribution, velocity distribution and vortex’s structure and evolution characteristics of wake region and other flow message were captured. The results showed that static pressure is asymmetric in impeller. And the reason for vortex forming in separation chamber was analyzed. The gas fraction distribution shows that gas phase mainly centers on the blade suction interface zone in the impeller. Performance prediction was carried out and performance curves were gotten. The performance experiment results was conducted and compared with prediction results. Large eddy simulation was accurate in predicting performance and interior flow characteristics.