Seamless WLAN channel migration

A WLAN system includes: an access point used the first channel or second channel to transmit the data, wherein the center frequency of the first channel is an original frequency and the center frequency of the second channel is a target frequency; and a client terminal. When the client terminal receives the data from the first channel, the access point and the client terminal are, respectively, operated by a first working frequency and second working frequency. Initially, both working frequencies are located at the original frequency. When the first channel is desired to be changed to the second channel, the first working frequency is added an off-set. The second working frequency adjusted is then followed so as to keep up with first working frequency. By adding the off-set to the working frequency repeatedly, the first and second working frequencies are shifted until both of them arrive at the target frequency.