Abstract: In terms of the active structure control problem of floating offshore wind turbines,an active sliding mode control method based on disturbance observation is proposed. The effectiveness of the proposed method is verified by using the wind turbine simulation tool FAST. On the premise that the second derivative of the disturbance is bounded,the stability of the derived disturbance observer is proved with bounded estimated errors,which means that the proposed disturbance observer is effective in estimating the matched and mismatched disturbances. A sliding mode controller is proposed based on an integral type sliding surface,where the finite time convergence and the closed-loop system stability are proved. Simulation results based on FAST show that the platform pitch angle and the tower-top displacement can be effectively reduced by using the proposed disturbance observer based sliding control method.Compared with the optimal passive TMD systems,the RMS values of the platform pitch angle and the tower-top displacement can be reduced by 11.88% and 13.56%,respectively.