Abstract: For underwater acoustic communication, the bandwidth is rigid restricted by transmission regions.In this paper, we research the relationship between the available bandwidth and the transmission distance, and propose a bandwidth balanced routing protocol, for underwater acoustic sensor networks (UASNs).When transmitting packets, it always selects the branch (from the next-hop node to the sink) whose available bandwidth is maximal, thus also reduce the congestion and keep balance of sensor nodes' energy consumption in some degree.The simulation result shows that the proposed protocol could implement the bandwidth balance of all branches in whole network, and the energy consumption balance, then prolongs the lifetime.