To solve the problems of lack of domestic fish pump products, long development cycle, and so on, taking the existing siphon fish pump as the research object, the CFD numerical calculation method was used to simulate the three-dimensional flow field in the fish pump under three different influent speeds (1 m/s, 1.5 m/s, 2 m/s), and the internal flow characteristics including velocity distribution, eddy viscosity, and pressure distribution were obtained. At the same time, the validity of the numerical calculation was verified by the experimental results. The results show that the pressure and flow velocity show an increasing trend with the increase of the inlet velocity, and the maximum pressure is mainly concentrated at the upper end of the outlet side of the fish suction pump, which will have a certain impact on the fish. In addition, the incoming flow collides with the wall of the fish suction pump to form reflux, and the reflux landing point is also prone to secondary damage to the fish. When the inlet velocity is 1m/s, the velocity of the bottom wall of the suction pump is 0.13m/s, and the experimental value is 0.15m/s. The difference between the experimental results and the numerical simulation results is about 13.3 %, which verifies the effectiveness of the calculation results. The research results provide an important basis and reference for the subsequent research and design of the fish suction pump.