In order to solve the problem that offshore small buoys only use single mode to communicate with the shore-based, a design method of offshore small buoy communication system combining Beidou short message and 5G communication is proposed. The communication link between the buoy and the shore-based command post is built to realize the remote information interaction function between the shore-based command post and the buoy, in which the hydrological and meteorological information collected by the buoy is uploaded to the shore-based command post through the optimal communication link in the current anchoring area, and the shore-based command post remotely controls the buoy to complete the corresponding tasks. Laboratory simulation results show that with the support of 5G communication and Beidou-3 global navigation satellite system technology, the system can enhance the robustness of small buoys in harsh sea conditions, improve the security of data transmission and improve the quality of communication.