The principle of radar altimeter and the working procedure of pulse radio altimeter were described in this thesis. The principle and design scheme of missile altitude control loop was put forward. The simulation and analyze was carried out to validate the relations between threshold of altimeter and the velocity and view-area of missile. The simulation shows that the threshold should be increased for stabilization of trajectory and be reduced when missile velocity is correspondingly big;If the view-area angle of seeker is changeless,the reducibility of the threshold of altimeter will avoid that remora keeps out the LOS of seeker.This helps to capture the target.