Abstract: MAC protocol provide node access and conflict avoidance service for underwater acoustic networks (UANs). Due to the high propagation delay and narrow bandwidth characteristics of underwater acoustic channel, the performance of MAC protocols in UAN is very low. In this paper, we propose a RS-MCAC protocol based on MACA-U protocol. The protocol utilizes the long channel reservation time that comes with the long propagation delay. When packet load is heavy, RS-MACA can send packets in parallel. A sender accepts the reservation of neighbor nodes in its own reservation stage. The neighbors do not need to spend extra time to reserve channel. Neighbors send their packet according to the time slot to avoid conflict. Performance evaluation is conducted by using NS3, and confirms that the proposed protocol significantly outperforms MACA-U protocol in terms of throughput.