Abstract: The passive radar system based on transmitters of opportunity is an old and special radar system. In recent years it is the potential particularity, such as covert and anti stealth, that the media has shown considerable interest in its status. Experts think that it will become a crucial trend of radar systems in the coming ten or twenty years. In this paper, three stages in the history of transmitters of opportunity based passive radar system is reviewed. At each stage some of technical characteristics are analyzed. In the end, some suggestions and related problems on investigation of passive radar system based on civil transmitters of opportunity are also discussed.