Tactical Data Links (TDLs) enable a military to exchange tactical information in a precise, efficient and timely manner. With the development of military information technologies, the data rate and the number of nodes participated in a network are experiencing explosive growth. Traditional TDLs medium access control (MAC) layer lacks sufficient flexibility, thus cannot meet the demand of real-time communications and efficiency. In this paper, a dynamic MAC (D-MAC) protocol is presented. With a Bayesian-estimation-based algorithm, the D-MAC adaptively selects the most high-priority packets to transmit. Meanwhile, the D-MAC divides all time-slots into fix slots and dynamic slots in order to increase the ratio of channel utilization. Those nodes with high-priority would occupy as more dynamic slots and transmit as more data as possible. The simulation results demonstrate that the D-MAC can efficiently use radio channel resources and achieve good performance.