In order to increase the network efficiency in TDMA protocols with the fixed guard time for underwater sensor networks, a new guard time setting approach between consecutive packets considering the uncertainty of the underwater acoustic propagation delay is proposed in this paper. By using a collision model of the two adjacent packets, the relationship of the guard time and the distances between the receiving node and the sending nodes under the given collision probability is analysed. The guard times can be varied with the distances between the receiving node and the sending nodes under the given collision probability, which can overcome the disadvantages of the previous TDMA protocols with the low network efficiency. Theoretical analysis and simulations show that this approach can reduce the idle time of channel and improve the efficiency of underwater acoustic sensor networks. The proposed approach is suitable for single hop underwater acoustic sensor networks based on continuous time allocation TDMA protocols and can be easy to extend to the tree-like hierarchical topology.