On the basis of relative motion theory, a midcourse guidance method of exoatmospheric interception was developed. The mechanism of this method is that treating the initial and modified trajectories as chief and deputy space objects respectively, modeling the dynamics of the deputy relative to the chief by the relative motion theory and solving the initial corrections through the final relative states. The universal relative motion model was provided and then the state transition matrix of relative motion under the influence of J₂ disturbance was derived by employing the geometry and the variation method. Lastly the midcourse guidance method using state transition matrix was proposed on this basis. Simulation example shows that the proposed method serves as an efficient support for midcourse guidance of interceptors in practice.