Net is an important part of cage. In order to solve the problem of numerical research on the movement deformation and stress of net, this paper proposes a mesh group improved method based on Morison equation and lumped mass method. The improved method keeps the volume and quality of the net unchanged before and after the mesh group, and the hydrodynamic force of the improved net is the same as that before the mesh group by adjusting the hydrodynamic coefficient. Through the numerical simulation of the circular net, and compared with the relevant research, the volume loss rate and horizontal drag force of the circular net are analyzed. The results show that: in the flow field of uniform velocity, the horizontal drag force of circular net increases with the increase of velocity, and then reaches the steady state, and the value is basically unchanged; increasing the bottom counterweight of net can effectively reduce the deformation of net, but with the increase of counterweight, the deformation inhibition effect is gradually decreasing. The improved mesh group method is accurate and effective, and has significant simplification effect and a good reference value for improving the hydrodynamic simulation efficiency and calculation accuracy of cage.