Abstract: Aiming at the low throughput of underwater acoustic communication networks due to the large propagation delay of underwater acoustic channel,a Medium Access Control(MAC) protocol for multi-node coordinated parallel transmission is proposed.The protocol is receiver-initiated,where the reserved nodes are grouped into several cooperative transmission cells based on the distance recognition to construct a centralized network architecture of multi-user MultipleInput Multiple-Output(MIMO).Moreover,the space-time encoding of the physical layer is adopted,and the transmission time of data packets is reasonably scheduled,so that the data packets sent by multi-user nodes in the same cell reach the receiver simultaneously within the predetermined time deviation,while the data packets from different cells arrive in the form of packet train,realizing interference-free and efficient parallel transmission of multi-user nodes.Simulation results show that compared with other two existing protocols,the designed protocol greatly reduces the average end-to-end delay and significantly improves the normalized network throughput by space-time multiplexing.