A cognitive radio system and method for clear and idle channel survey in Wireless Local Area Network (WLAN) are disclosed. Through computing average value of Received-Signal-Strength-Indicator (RSSI_AVG) in each channel, clear channels without interference from other RF devices can be distinguished in the present system and method. Based on an actual WLAN setup, a further AP has a larger possibility to be an idle and clear AP. In the present system and method, searching available channels from furthest AP to nearest AP therefore helps to reduce time of channel survey through repeated rises of signal strength threshold.