Abstract: A model of channel access state transition for VHF air/ground datalink, which uses the continuous time Markov chain and the CSMA matrix analyze method, is proposed in this paper. The throughput and data package delay of datalink under various user access laws and user numbers of sending data are calculated to analyze the effects of air/ground report channel loading and channel transmission detection time on the performance of VHF datalink. The results show that the user numbers, retransmission ratio and transmission detection time have significant effects on the performance of VHF datalink.