In this paper, we simulate the tri-message time synchronization protocol within the cluster sub net for the MAC layer of underwater acoustic network. Distinguished with the traditional TDMA protocols, all sub nodes are time consistent with the cluster node and the sub nodes can be quickly netted in the improved TDMA protocol. We established a network simulation with a bunch of 6 nodes in OPNET software. In this scenario, we run two sets of the improved TDMA protocol and the traditional TDMA protocol of the MAC layer. The simulation result gives time synchronization effect, end-to-end delay, throughput, packet loss rate of each of both protocols. With comparison and analysis, we verify that the improved TDMA protocol has better network performance than traditional TDMA protocol.