A two-way fluid structure interaction (FSI) of CFD–FEA numerical method is adopted to predict ship motions, wave loads and hydroelastic responses in regular waves. In the FSI algorithm, the external fluid loads obtained from the CFD model and the motions and structural deformations obtained from the FEA model are exchanged between each other in an iteratively staggered implicit coupling regime in time domain. The motions, vertical accelerations and wave loads including springing and whipping responses of a flexible S175 container ship under different regular wave conditions predicted by the FSI co-simulation method are systematically analyzed. High order hull girder vertical vibrations of up to 12th order harmonic springing and up to 4-node whipping responses have been successfully reproduced, which indicates that the CFD–FEA co-simulation method is reliable and capable in simulating ship nonlinear hydroelastic responses in waves. Moreover, some complex flow phenomena such as slamming, green water on deck, wave breaking and splashing have also been well reproduced from the CFD simulation. It is believed that the present FSI method will exhibit significant advantage over the traditional ways used for ship seakeeping and hydroelasticity analyses.