To solve the uncertain projectile velocity disturbance and nonlinear friction problems of the stabilizer, combining with sliding mode control and observer theory, a second-order sliding mode control based on nonlinear disturbance observer is proposed. A differential tracker is adopted to estimate the derivation of the state value, and furthermore, the first disturbance is acquired. Through the coordinate transformation, disturbance in the system is normalized, which simplifies the controller design. Secondly, an observer is designed based on the differential tracker, and the controller aims to achieve precise estimation of normalized disturbance. Finally, a second-order sliding mode control is adopted. The simulation results show that the new nonlinear disturbance observer estimates accurately and responses quickly, and the proposed second-order sliding mode control based on disturbance observer is able to isolate the disturbance of the missile motion and nonlinear friction of the seeker stabilized platform. The method further improves the robustness and tracking accuracy of the stabilization.