In order to monitor bred fish in an offshore aquaculture cage farm and the aquatic environment surrounding the farm daily, a centralized remote monitoring system was developed. The system is composed of a group of Remote Detecting Nodes (RDNs) and a Local Monitor and Control Center (LMCC). RDNs based on the techniques of circular multi-beam detection and multi-path signal process were placed in the center of the cages to scan the inner space and measure the aquatic environment. LMCC based on LabVIEW was situated in the office to manage all the RNDs remotely and show fish activities and the state of net clothes. The communication between LMCC and RDNs used GPRS wireless data transmission technology. The system prototype was tested in the field of offshore aquaculture cage farm in Quanzhou, China. Results show that the system is reasonable design, simple operation, easy placement, and robust under different environmental conditions. It can monitor the aquatic environment and fish activities remotely and real-timely, provide users with a variety of culture information such as fish amount, fish growth conditions and culture safety in the offshore fish cage, and give an alarm in time when net clothing damage or fish escape.