Due to underwater acoustic sensor networks' (UWASN) broad application prospects in recent years, it catches people's attention gradually. However, due to restrictive underwater environment, acoustic sensor network has some uniqueness. For example, unlike two-dimensional terrestrial sensor networks, underwater acoustic sensor network is three-dimensional. Today, the most existing Medium Access Control (MAC) protocol are designed for the two-dimensional wireless sensor networks, instead of wireless networks based on 3D rarely. In this paper, we proposed a three-dimensional network topology based underwater acoustic sensor network MAC protocol. Nodes in the network will be placed to tree structure according to this protocol. This protocol uses the relationship between the child and its parent, to achieve dynamic three-dimensional networks through dynamic node algorithm. In addition, we analyze and effectively resolved the collision between the nodes, greatly improving the channel utilization. Through the simulation software contrast experimental results of different protocols on the same network topology, it confirms that our protocol can effectively save a lot of energy.