Variable-speed constant-frequency wind power generation technology has been widely used in actual wind turbines. A typical issue is the Maximum Power Point Tracking (MPPT). This paper designs a Perturbation Observer Based Sliding Mode Control (POSMC) scheme for MPPT of Doubly-Fed Induction Generator (DFIG). It combines the perturbation observer and Sliding-Mode Control (SMC) to significantly enhance the robustness. Firstly, the perturbation observer estimates the perturbation (generator nonlinearities, parameter uncertainties, and random wind speed) online. Then, the perturbation estimation is compensated in the real-time by SMC, such that a global control consistency and improved robustness can be achieved under different operation conditions. Finally, POSMC does not require an accurate DFIG model while only the rotor angular velocity and stator reactive power needs to be measured, thus it is easy to implement. Three case studies are undertaken, namely, step wind speed, random wind speed and uncertain generator parameters. Simulation results show that, compared with Vector Control (VC), Feedback Linearization Control (FLC) and SMC, POSMC can capture the maximum wind power under various operation conditions and can provide the strongest robustness. This work is supported by Science and Technology Project of China Southern Power Grid Company Ltd. (No. GDKJXM20180576).