In recent years, the fishery production has been drastically decreased in Taiwan, mainly due to overfishing and coast pollutions, therefore fishermen and corporation are encouraged by government to invest in ocean cage aquaculture. However the high price fishes in the net cage are often coveted, so the incidences of fish stealing and net cage breaking were found occasionally, which cause great economical loss. Security guards or visual monitoring system has limited effect especially in the night when these intrusions occur. This study is based on acoustic measure to build a net cage alarm system, which includes the sonobuoy and monitor station on land. The sonobuoy is a passive sonar that collects the sounds near the net cage and transmits the suspected signal to the monitor station. The signals were analyzed by the control program on the personal computer in the monitor station, and the alarms at different stages could be activated by the sound levels and durations of the analyzed signals.