The influence of nonlinear hydrostatic restoring force on the hydrodynamic characteristics of offshore floating Spar wind turbines is studied. The nonlinear hydrostatic restoring stiffness coefficient is derived by the analytical method and verified by the linear model in AQWA. Considering the nonlinear hydrostatic restoring stiffness coefficient, the time-domain analysis of the Spar floating wind turbine is carried out by programming, the results are compared with the motion response under linear hydrostatic restoring force, and the hydrodynamic characteristics of the Spar floating wind turbine and the influence of the hydrostatic nonlinearity are analyzed. The results show that: the fluid static nonlinearity is of a significant impact on the heave response of the floating wind turbine, and this impact increases with the increase of the rotating motion of the floating wind turbine; under the extreme sea conditions, the difference in the heave, roll and pitch response caused by the hydrostatic nonlinearity is within 16% to 40%, showing that the nonlinear hydrostatic force should be taken seriously in the motion response.