In the paper, the QoS support for Media Access Control (MAC) layer in wireless LAN is researched. On a basis of static parameter setting used in EDCF’s backoff mechanism, in combination with network load, the compete window and backoff time with priority of different services can be dynamically adjusted to select the access mode to the channel in order of priority. The simulation shows that the dynamic adjustment has better performance in network throughout, time delay and collision rate than the mentioned static parameter setting.