Evaluation of dynamic responses under extreme environmental conditions is important for the structural design of offshore wind turbines. Previously, a modified environmental contour method has been proposed to estimate extreme responses. In the method, the joint distribution of environmental variables near the cut-out wind speed is used to derive the critical environmental conditions for a specified return period, and the turbulence intensity (TI) of wind is assumed to be a deterministic value. To address more realistic wind conditions, this paper considers the turbulence intensity as a stochastic variable and investigates the impact on the modified environmental contour. Aerodynamic simulations are run over a range of mean wind speeds at the hub height from 9–25 m/s and turbulence levels between 9%–15%. Dynamic responses of a monopile offshore wind turbine under extreme conditions were studied, and the importance of considering the uncertainties associated with wind turbulence is highlighted. A case of evaluating the extreme response for 50-year environmental contour is given as an example of including TI as an extra variant in environmental contour method. The result is compared with traditional method in which TI is set as a constant of 15%. It shows that taking TI into consideration based on probabilistic method produces a lower extreme response prediction.