Abstract: A trajectory tracking guidance law is proposed based on the theory of sliding mode variable structure control, which can help solve the problem of trajectory tracking for air-defense missile. Firstly, the missile mass model is linearised according to the coordinate variable instead of the time variable. Secondly, the sliding mode surface and the guidance commond are designed based on the theory of sliding mode variable structure control for the single input linear system, and the principle of the guidance system is given. Finally the guidance law is evaluated with certain wind disturbation, in terms of the advantage of the model, the guidance law’s abilities to deal with disturbation and robustness and then compared with the guidance law of the linear quadratic regulator (LQR). Results show that the model used in this article is better than the traditional one which is linearised according to the variable of time, and the sliding mode variable structure control guidance law is better than the LQR guidance law in the robustness to uncertain model errors.