An innovation in aquaculture fisheries around the world is in progress. More and more fish cages have been put into use to meet the needs of human for protein. However, the fish cage shows violent dynamic response and structural destruction while suffering to unpredictable marine environment. As the main component of the cage, floating collar plays an important role in providing buoyancy and ensuring the shape of cage. Thus, the dynamic response of the floating collar and the variation of flow field around the floating collar under extreme wave condition were studied in this paper. Referring to the previous literature, considering the motion form of the floating collar under waves, only the heave and pitch motions of the floating collar were obtained. Results obtained by Computational Fluid Dynamics method were compared with that obtained by potential flow theory. We found that the viscosity of the water has greater influence on heave motion of the floating collar rather than the pitch motion. At the same time, the flow field surrounding the floating collar was analyzed, and an overtopping phenomenon on both sides of the floating collar along the wave propagation direction was observed when the wave was passing through the floating collar.