Due to mission requirements, unmanned craft vehicle(USV) usually operate in the form of formation cooperation and exchange data through wireless ad hoc networks. Due to the influence of ocean waves and other factors, the channel transmission loss of maritime ad hoc networks is usually in a dynamic state. The existing backoff algorithm of MAC protocol in ad hoc networks cannot distinguish between packet collision and packet loss in a dynamic maritime environment, resulting in the decline of reliability and stability. Therefore, this paper proposes an adaptive minimum contention window backoff algorithm based on channel monitoring. The algorithm estimates the channel state by sensing the number of adjacent contention nodes, reduces the channel collision probability and retransmission times, and improves the reliability and stability of the network as a whole. Simulation results show that compared with the classical BEB algorithm, the throughput and fairness of the improved algorithm are increased by 28.67% and 62.00% respectively, and the end-to-end delay and packet loss rate are reduced by 2.84% and 15.10% respectively.