To ensure the normal service of offshore floating wind turbine under complex environmental loads, such as wind and wave, it is necessary to evaluate the long-term extreme response of offshore floating wind turbine. Through the joint probability distribution of measured wind and wave, the combination of environmental conditions is obtained by the environmental contour methods based on the IFORM and the ISORM, and the short-term response is obtained by simulations. Combined with the Gumbel extreme value, the long-term extreme response of the wind turbine is calculated, and the extreme response analysis of the 50-year return period of the offshore floating wind turbine is realized. The results show that under the combined effect of wind and wave, with the increase of the mean wind speed, platform surge motion, bladed root out-of-plane bending moment, and tower base fore-aft bending moment first increase and then decrease; as the significant wave height increases, the maximum and mean values of platform surge motion and the maximum value of tower base fore-aft bending moment also increase. Compared with the environmental contour method based on IFORM, the environmental contour method based on ISORM can cover more combinations of environmental conditions and obtain greater long-term extreme response, which improves the safety of the wind turbine structure design.