Abstract: Ad hoc network is composed of lots of distributed nodes, but the lifetime of nodes is short. In this paper, a three−clock multi−priority N−CSMA protocol based on variable collision length is proposed to improve the transmission efficiency of nodes.It is introduced multi−priority mechanism and reduced the time slot length of information packet collision to meet the QoS requirements of different priority services. So it is improved the throughput of the system, effectively alleviate channel congestion and the system performance of wireless ad hoc network. Finally, the accuracy of this protocol is verified by simulation experiments and compared with other protocols to prove the superiority of this protocol.