In order to reduce the effect of the delay characteristics of missile autopilot on guidance precision and the terminal guidance time of actual war is short. The mathematical model considering the finite time convergence for the dynamics of the missile autipilot is built. The guidance law is designed based on sliding mode control theory. The designed guidance law is proved stable in the system. A hyperbolic tangent function is used to improve the guidance law in order to reduces the chattering of guidance law. The simulation shows that the design of guidance law can fastly track the target movement in a limited time and maintain a high guidance precision when the target is non-maneuvering or maneuvering.