Abstract: Based on the nonlinear mooring loads induced by the deformations of mooring lines, including sketching, bending and torsion effects, the dynamic responses of floating offshore wind turbines (FOWTs) were analyzed. By considering the nonlinear geometric deformations, a stretching-bending-torsion coupled dynamic analysis model for a mooring system was proposed. Euler parameters were used for the transformation between the global and local coordinate systems to avoid the occurrence of singularity. The dynamic analysis model of the FOWT was established by considering the nonlinear mooring loads, hydrodynamics, aerodynamics as well as the mechanical system of a wind turbine. Based on this nonlinear formulation, a numerical program of the dynamic response of FOWT was developed. The OC3 Hywind Spar with a 5 MW baseline wind turbine was selected as a sample platform. The dynamic responses under both the operating and extreme sea states were simulated. The results were compared with those calculated based on the catenary theory. By considering the dynamic effect of mooring lines, the wave frequency response of tension is amplified, and several peaks in the range of high frequencies occur. Those responses will affect the fatigue life of mooring lines.