SST k-ω turbulence model was adopted to simulate the flow characteristics of the axial-flow pump in the following four states, non-cavitation in clean water, non-cavitation in sandy water, critical cavitation in clean water and critical cavitation in sandy water. The influence of sediment concentration and cavitation on the flow characteristics of the pump was analyzed, and the unstable operating area of the axial-flow pump in different working conditions was determined. The results show that on the blade working surface, the axial flow coefficients are distributed around -1 in the non-cavitation and critical cavitation states, and the radial flow coefficients and the absolute flow coefficients are distributed in the non-cavitation state near 0. 6 and 1. 2, respectively. In the critical cavitation state, the radial flow coefficient and the absolute flow coefficient are distributed around 1. The flow coefficients at the inlet and outlet of the back of the blade are the most sensitive. Cavitation on the impeller, which has a great impact on the flow coefficient, can cause fluctuations of flow coefficients in the blade back face especially after adding sediment. Thus, the inlet and outlet on the back face of impeller are unstable due to the impacts of sediment and cavitation. When cavitation does not occur, the flow state in the impeller is basically not changed by the sediment concentration. When cavitation occurs in the pump, a flow separation vortex appears at the tail end of the blade. After adding sediment, the vortex range at the tail end can increase, and the internal flow state of the impeller is thus changed. It shows that both cavitation and sediment are the main factors affecting the flow characteristics in the impeller.