Abstract: Due to its special role in adjusting internal flow structure, a reflux hole is a key device that affects the performance of outer recirculation self-priming centrifugal pumps. To examine its effects, the internal flows have been numerically simulated in this study for an efficient self-priming sewage pump of ZW type with a reflux hole connecting the gas-liquid separation chamber to the volute. Results show that this reflux hole had a significant effect on the internal flow and pump performance. It produced a circular flow and a considerable increase in head loss and hence reduced pumping lift significantly. As the pump flow rate was varied, the pressure distribution at the chamber and volute sides of the hole was changed in a regular way, and the flow patterns near the hole inlet and outlet also changed. A change in the hole cross-sectional area caused a change in the pressure drop from the hole inlet to outlet and a change in the hole flow rate, thus modifying the external characteristics of the pump.