Abstract A numerical approach is proposed to simulate the hydrodynamic characteristics of a net cage in a steady current. The numerical approach is based on the joint use of the porous-media fluid model and the lumped-mass mechanical model. The deformation of a net cage can be calculated using the lumped-mass mechanical model and the flow field around a fishing net can be simulated using the porous-media fluid model. Using an appropriate iterative scheme, the coupled fluid–structure interaction between the flow and net cage can be solved, and the steady flow field inside and around a net cage can be obtained. Using the proposed numerical approach, numerical simulations of flow through a single net cage in different currents, single net cage with different weight modes and multiple net cages are investigated. Validation of the numerical model is conducted by comparing the numerical results with the corresponding physical model measurements. The comparisons demonstrate that both the configuration of the net chamber and the flow-velocity results are in accordance with those of the corresponding physical model tests. It was determined that more accurate results can be obtained by considering the coupled fluid–structure interaction between the flow and the net cage.