Due to the long propagation delay and the low data transmission rate, many transmissions of control packets will occupy a lot of channel resources, power consumption and transmission time for underwater acoustic sensor networks (UWSNs). In this paper, we propose a receiver-initiated MAC protocol with packet train for UWSNs called multi-receiver MAC (MR-MAC) protocol. Our protocol will increase of the throughput and reduce of the transmission of control packet for handshaking. Our MR-MAC protocol can make more than two nodes to communicate in one handshake held by a main receiver. By scheduling the packet transmission time, the data packet will be sent in a packet train manner and the receiver can receive data packet without collision. Our simulation results show that our protocol can increase the throughput and decrease the packet delay than the other existing MAC protocols.