In this paper, the maximum power point tracking (MPPT) control problem for wind turbine systems with multi-uncertainties is considered. A tracking controller of optimal tip speed ratio is proposed based on fully actuated system (FAS) approach. First, the dynamics of wind turbines in the region below rated wind speed is represented in FAS model. Then, with the analysis of multi-uncertainties in operation of wind turbines, an extended state observer (ESO) is designed to estimate the uncertainties. A controller with uncertainty-compensation ability is proposed based on FAS theory to obtain a closed-loop tracking system of rotor speed with fully actuated characteristics. Theorems are given to show the advantages on stability analysis, parameters selection under constraints and engineering regulation of FAS approach in MPPT control of wind turbines. Finally, an example of 5MW wind turbine of NREL is given for simulation. The results indicate the effectiveness of the proposed approach for MPPT control.