Large-scale multi-AP WMAPNs with a high density of users and access points (APs) have emerged widely in various hotspots, where inter-cell interference is detrimental to the performance of WLANs. In order to mitigate the interference, channel assignment scheme has been a major research topic in recent years. In this paper, based on Bianchi's throughput performance evaluation, we model a channel assignment problem for WLAN with Multiple APs (WLMAPN) not only to maximize the overall system throughput, but also to achieve the fairness among APs, where the inter-cell interference is measured in terms of the throughput reduction. In order to cope with this problem, channel assignment based on the Order of Throughput Reduction (CAOTR) scheme is proposed with low computation complexity, using local search. Finally, extensive simulation results illustrate that the proposed CAOTR scheme significantly enhances the overall throughput and guarantees the fairness of APs in density WMAPN systems.