Traditional missile guidance law is difficult to adapt to the complex operational environment, but the variable structure control has the advantages of fast response, insensitive to parameter variation and disturbance, none on-line identification in system, simple physical realization and so on. In this paper, we take the medium-range air-to-air missile fire control as the research object and make use of the sliding mode variable structure control algorithm to establish the medium-range air-to-air missile variable structure guidance law and then we calculate the change rates' expected value, as a result, the dynamic model and the kinematic model of the missile are combined so that the variable structure guidance model is formed.