The principle and the existing problems of radio amendment instruction consisted in the classical mediumrange airtoair intercept missile are described. Then a new method of variable period midguidance instruction is proposed. A variable period instruction model is established based on the Current Statistical Model (CSM) of target. The predicted target motion value is introduced into midguidance instruction,and the period of midguidance instruction is changed according to the estimated remaining time of midguidance. The simulation result indicates that the modified instruction model can improve the tracking efficiency of seeker and limit the error of targets designation angle in intercepting and capturing.