As for the multi-user OFDM system, we propose a cross layer resource allocation algorithm 1n this paper which joints MAC layer and PHY layer. The proposed algorithm first use a ?nite-state Markov chain to analyze the users’ queue status. Secondly, we obtain the maximum rate and the minimum rate of PHY layer according to the users’ QOS constrains. Finally, we joint the time-varied channel state information of PHY layer and the former rate constraints to implement the dynamic resource allocation. The results show that Compared with the resource allocation algorithm without the maximum rate constraint, the proposed algorithm can not only meet users’ QOS requirements (packets loss rate), but also can save some power, which just improves the performance of the system.