During the last decades, there are numerous researches in the field of wireless local area networks (WLANs). A large number of these researches focus on performance analysis of IEEE 802.11 Medium Access Control (MAC) protocols. And the most representative model for MAC mechanism in WLANs is the two-dimensional Markov model, which has been proven to be an accurate model to analyze the performance of WLANs, in the assumption of finite number of terminals and ideal channel conditions. But this two-dimensional Markov model is rather complex, as far as computation and analysis are concerned. Recently, a novel semi-Markov process model for IEEE 802.11 MAC protocols has been put forward to mitigate the complexity of analyzing the performance of WLANs. However, this semi-Markov model has some defects, such as not taking maximum retransmission number into account. In this paper, we propose an advanced semi-Markov process (ASMP) model, which not only withholds the merits of semi-Markov chain model, but also eliminates its shortcomings, thus more accurately calculating the network parameters of WLANs. Simulation results show that our proposed model achieves accurate results with less complexity, and is suitable for evaluating the performance of the MAC protocols.