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In standard access control mechanism, contention window is designed according to the number of collision, and ignoring the current congestion information in the network. Consequently, terminal cannot adjust the contention window timely according to the current network load. In order to solve this problem, slot utilization and the probability g are proposed in this paper to estimate the network congestion information. Then the back-off algorithm in standard protocol is improved and a novel P-probability access control mechanism (PACM) is proposed to improve the distributed access control mechanism which is aware of congestion state in 3G wireless networks. In PACM, the network congestion information is applied to basic access control, which assigns the contention window after sending data frames. Simulation results show that the probability of frame collision may fall and the throughput of the whole 3G networks can be enhanced.