The present invention proposes a device and method for detecting the channel state. The detecting device first monitors a channel within a predetermined duration so as to receive a plurality of signals transmitted on the channel, and then determines whether part of the plurality of signals possess periodicity. If part of the plurality of signals possesses periodicity, then the detecting device determines that the channel is interfered. Even if there is no or less data traffic over one or more wireless networks which occupy the channel, the detecting device and method provided by the present invention are capable of detecting the existence of the one or more wireless networks, and determine that the channel is interfered, so as to avoid potential interference. In addition, the cost to detect whether a channel is interfered will be reduced effectively by using the detecting device and the detecting method according to embodiments of the present invention.