In this paper, the problems of actuator fault detection and reconstruction based on observers for a kind of uncertain nonlinear system are discussed. Under some assumptions, a residual which is robust to disturbance but sensitive to actuator fault is produced by designing a kind of full-order sliding model observer. Then the residual can be used to detect the actuator fault. Under the same assumptions, a reduced-order observer which can eliminate the influences of unknown disturbance and actuator fault directly is developed. A method to reconstruct the actuator fault is provided based on the reduced-order observer. After this, a kind of numerical solution for the fault reconstruction is proposed by using the some numerical methods. Finally, the simulation results on a real model show the effectiveness of the proposed method.