With the number of mobile devices increasing rapidly, wireless local area networks (WLANs) based on 802.11 standard, are becoming more and more popular. All the stations (STA) including access point (AP) have the same traffic load because the 802.11 standard adopts equal throughput design model. But in real life, downlink traffic is almost always larger than uplink traffic. The design model of WLANs will limit downlink packets transmitting, so the asymmetric traffic loads between uplink and downlink is an important problem to solve. This paper gives a solution to solve this problem by adaptively adjusting minimum contention window (CWmin) of AP and STA based on the number of active stations (STAs).