The invention provides a maximum power tracking method of a large directly-driven wind turbine. The estimation value of the air power torque of a wind wheel and the measured value of the rotation speed of the wind wheel (or the rotation speed of a power generator) are provided as the input quantity of the maximum power tracking algorithm, the method doses not depend on accurate fan model and environment data, and wind speed measurement is not required. Through adopting the ratio coefficient of self-adaptation regulation, the rapid and accurate searching of the maximum power point of the variable step length is realized.