With the growing application diversification in Ad Hoc networks, it is particularly important to support prioritization of traffic and ensure that critical data can be delivered more reliably and faster in an emergency scene. We mainly study on the improvement of statistics based multi-priority medium access control (MAC) protocol, including the channel occupancy statistic and the backoff mechanism. We conduct comprehensive simulations based on the OPNET Modeler and account for the success rate results with theoretical analysis. The simulation results show that the proposed algorithm can simultaneously guarantee high success rate and low delay for high-priority packets.