Considering the internal parameter perturbation and external disturbances of Direct-driven Permanent Magnet Synchronous Generator (D-PMSG) wind power system, an improved integral Sliding Mode Control (SMC) method based on the Nonlinear Extended State Observer (NLESO) is proposed to achieve Maximum Power Point Tracking (MPPT). NLESO is designed by nonlinear smoothing function, and the disturbances and uncertainty are estimated and compensated by NLESO, which improves the tracking ability of the speed. The sliding mode approach rate is designed by nonlinear smooth function which eliminates the high frequency buffeting phenomenon in the traditional SMC, and the stability of the SMC is analyzed based on the Lyapunov principle. The simulation results show that compared with the traditional PI control, the proposed method not only has fast response speed, no overshoot and no chattering, but also has good disturbance rejection ability and can achieve MPPT rapidly in the case of wind speed mutation, so it has great application prospects in the field of MPPT control of wind power system. This work is supported by Key Project of Hunan Education Department (No. 17A006).