The random multiple access protocol is of great significance to the quality of service（QoS）of the vehicular ad hoc networks. Due to the highspeed mobility of the vehicle nodes in the network, the network topology changes frequently. The fixed media access control protocol will limit the overall QoS of the highdynamic ad hoc network. In order to solve this problem, based on the CSMA/CA series protocol backoff algorithm, a competitive window adaptive backoff algorithm (NCWCOCT) is proposed, which is based on vehicle node density, channel occupancy factor and collision threshold. Firstly, in order to minimize the collision probability, a onedimensional Markov model is established based on the vehicle node density and the competition window value, and then the objective function is constructed. Then, the channel occupation factor and the optimal conflict threshold are proposed to realize adaptive backoff of the competition window aiming at optimizing the communication quality of vehicular ad hoc networks. Simulation results show that NCWCOCT is compared with similar DCW algorithm and IMBEB algorithm, the throughput performance is improved by an average of 1164% and 677%; the packet loss rate performance is reduced by an average of 1946% and 1329% respectively.