To research the principle of gas-liquid flow in the process of fish pumping and the effect of pumping pressure on the fish sucking speed of a vacuum fish pump, the vacuum fish pump of a deep-sea aquaculture platform is taken as the research object. Based on the VOF two-phase flow model, the flow process in the internal passage of the fish collecting device of the vacuum fish pump is numerically simulated. The numerical results show that, under certain pumping pressure, water continuously enters the fish collecting tank, and a foam gas-liquid two-phase mixture composed of water and a large number of air masses is produced. Bubbles continuously gather and move towards the air extraction port until they flow out of the air extraction port, the time for filling the collector with water is very short. The start-up velocity at the inlet of the pipe increases with the decrease of the pumping pressure, and then the velocity fluctuation decreases. In the pressure range of -40kPa ~ -20 kPa, after t=7s, the velocity at the inlet of the pipe gradually becomes stable. When the pumping pressure is ≥ -15kPa, the water pipe has a backflow phenomenon, and the water sloshes back and forth along the pipe. Therefore, when the pumping pressure is in the range of - 40 kPa to - 20 kPa, it can ensure that the fish water mixture can be smoothly pumped into the fish collector. The numerical simulation results can provide theoretical support for the design and optimization of pumping pressure.