For a class of uncertain nonlinear systems, the control problem under the condition that every order of the systems is affected by unknown disturbance is investigated. Firstly, a n-orders super twisting disturbance observer is introduced, which estimates the disturbances of the systems. Then, based on the structure of the systems and the sliding mode control technique, a sliding surface with integral function is designed. Combineing with the second order fast terminal sliding mode control theory, a sliding mode controller based on the proposed disturbance observer is presented. In order to alleviate the chattering caused by the sign function, the sigmoid function is employed. And the stability of the closed-loop system is proved by the Lyapunov theory. Finally, the method is applied to the simulaton for controlling the magnetic levitation system, and the results show the effectiveness of the proposed method.