A fish cage is usually composed of steel columns, buoys, flexible nets and mooring lines. Elastic deformations will occur on these components under waves. The effects of these deformations on the global dynamic responses of the fish cage are still unclear. In this paper, a method considering hydroelastic deformation and a method considering the cage as rigid structure are employed to evaluate the hydrodynamic performance of a fish cage structure. The wave forces acting on these slender components are calculated based on Morison formula. The displacement of the cage and tension forces in the mooring lines obtained from these two methods are compared and the effects of hydroelastic deformation are discussed.