It is important to tactical use of MMW (Millimeter Wave) active decoys. According to the features of MMW radar guided missile, two acting modes of MMW radar decoy are put forward for scanning-search and target-tracking stage. The action mechanism and disturbance effect are analyzed, as well as key technology including the disturbance frequency, disturbance power and multi-decoy mutual disturbance. The analysis shows that according to the situation of battle field throwing the MMW radar decoy reasonably can protect target effectively.