The reliable design of offshore aquaculture structures (OAS) for fish farming in the open ocean is vital to the marine aquaculture industry in the future. However, the lack of easy-to-access numerical tools for the dynamic analysis of OAS challenges the sustainable development of marine aquaculture. This article presents a newly developed numerical library under an open-source, finite-element analysis code, Code_Aster, enabling the dynamic analysis of OAS. A numerical model of OAS is first developed using the present numerical library and then validated against experiments. The validation shows a good agreement in terms of structural motions and tensions in mooring lines. After that, the dynamic responses of this model are analyzed subjected to irregular waves and current conditions from field measurements on an offshore fish farm site. The results indicate that a negative mean pitch angle will occur when the current velocity is large.