Sewage pumps are one of the most popular types of fluid machinery in municipal, industrial, and environmental engineering and many other fields. The influence of tip clearance size on the pump performance and internal flow field distribution within a sewage pump was investigated in this study based on numerical calculation and test verification. Three models with different tip clearances were obtained by adjusting the assembly structure and simulated by the computational fluid dynamics software to solve the Navier–Stokes equations for three-dimensional steady flow. The accuracy of numerical methods was verified by comparison with the experimental results. The sewage pump performance under different tip clearances was then compared and analyzed. The structure and propagation of the vortices inside the impeller caused by the leakage flow with different tip gaps were also analyzed. The leakage flow in the impeller couples with the cover reflection and secondary flows, forming different vortex structures in various locations of the impeller channel. These vortex structures cause large hydraulic losses near the blade suction surfaces. This finding could provide useful insights for the improvement of the hydraulic performance and operational stability of semi-open sewage pumps.