Abstract: Considering the characteristic of high energy consumption in underwater acoustic sensor networks, an Energy-efficient Routing protocol Based on Spatial Wakeup (ERBSW) is presented. It divides three dimensional network space into wakeup layers and sleep layers, each node makes local decision on whether to wake up or to sleep according to its current depth. In addition, ERBSW gets wakeup neighbor sets by broadcasting Hello packets periodically, and delivers data from nodes in higher wakeup layer to nodes in lower wakeup layer, which avoids energy consumption caused by idle listening and unnecessary data reception of redundant nodes. Compared with the Vector-Based Forwarding (VBF) protocol, simulation tests show that the proposed protocol can save energy cost by about 16%~48% in various network density.