Aiming at the fault of silent sentry radar system in war field，a kind of method based on portable passive radar is introduced in this paper. First of all，target can be ranged by triangle-locating method. Secondly，detection range is improved by coherent processed the reflected signals. Finally，multi-target detection can be realized by time delay match-scanning in time domain，instead of space-scanning by antenna array. The calculated results show that the detected range can be 70 km and the radar can detect 20 objects at the same time. The way is valuable to country and cities emphases recovery system；by its simplicity and cheapness.