The upper drainage method is often used in drum type washing machines by using a miniature drainage pump. With the decrease in water level, the pump undergoes in the state of gas-liquid mixed transportation, empty discharge, at the end of drainage and produce unsteady flow. This unstable flow not only results in the deterioration of pump performance but also induces larger vibration and noise, which affects the quality of the equipment seriously. In order to explore the internal flow characteristics of the drainage pump during the empty discharge period, pump performance test was processed under pure water condition as well as under different air void fraction. The results show that head drop slowly with the increase of flow rate and the pump exhibits low efficiency. But the efficiency of the model pump is in good agreement with the value recommended by the manufacturer and the influence of gas on the drainage pump is smaller when the gas content is 1%. As the gas content increases, the slope of the flow-head descending curve becomes larger, and the influence of gas content on the pump performance increases with the increase of flow rate. Besides, the overall and local results of internal flow field and gas phase distribution are obtained and analyzed.