Abstract: The dynamic characteristics of the semi-submersible floating wind turbine are researched in this paper. The dynamic equation and the transfer function considering viscous damping are derived. The effect of viscosity on dynamic response and the calculation method of viscous damping are studied. The advantages and disadvantages of the additional damping matrix method and the Morison method are compared. And a hybrid method considering viscous damping is proposed. On this basis,the dynamic response of the semi-submersible floating wind turbine considering the full coupling of aerodynamics,hydrodynamics and mooring is analyzed. The results show that the viscous effect mainly affects the response value near the resonance period and it cannot be ignored in numerical analysis. The additional damping matrix method is insufficient when considering the viscous damping of motion in the horizontal plane and it cannot consider the effect of viscosity on the resonance period. The Morison method is deficient in calculating the heave and pitch response considering the viscous damping effect. The hybrid method is a more effective method for hydrodynamic calculations considering the viscous effect. The heave and pitch responses of the semi-submersible floating wind turbine are mainly controlled by waves,while the motion responses in the horizontal plane are affected by the combined action of wind,wave and current. The most dangerous conditions for motion and acceleration of the floating wind turbine may occur during power generation. The maximum tension of the anchor chain occurs under extreme environmental conditions.