Cruise farming by deep-sea farming vessels is an important direction in the development of modern marine fisheries. Numerical research on aquaculture tanks can provide evidence for evaluating fish-friendliness in the aquaculture tanks of the vessels. To study the characteristics of the vortex flow field in the tanks of a deep-sea aquaculture vessel and analyze the fish-friendliness of the aquaculture tanks, a numerical aquaculture tank with a water circulation system was developed based on the secondary development of the open-source software OpenFOAM. The relationship between the characteristics of the vortex flow field and angles of inlets is analyzed using the control variable method, as well as the influence of the circulating flow rate. The results show that the flow velocity in a water-circulating aquaculture tank is analyzed by statistical methods. The free surface vortex phenomenon of real aquaculture tanks is reproduced in numerical simulation. Under a fixed flow rate, the overall velocity of the vortex flow field is greatly improved compared with the non-vortex flow field. Plugging of the inlets’ flow is an important factor for the generation of the vortex, and in terms of velocity, the vortex flow field has better fish-friendliness. The numerical results manifested that the flow velocity is related to the circulating flow rate and the angle of inlets, which indicated that the flow field in the aquaculture tanks can be adjusted by adjusting the flow rate or angle of inlets.