VoIP over WLAN which is called VoWLAN is an application of VoIP based on WLAN. This article mainly researches the PCF polling scheme in the IEEE802.11 Protocol. An improved PCF polling scheme has been proposed to meet the requirement of real-time business. The improved PCF polling scheme is based on Superpoll algorithm, it uses the concept of priority service and combines with the PCF uplink algorithm. In the improved PCF scheme, the VoIP station and VoIP packet has a higher priority, they can get continuous services. According to the experiment of simulation software, the improved PCF polling scheme can support the real time business such as VoIP effectively, the time delay of the VoIP packet has been reduced, and the session quality of the VoWLAN has evidently improved.