The present application provides an on-line fault detection device, method and system. The device comprises a reading unit, a judgment unit, a detection unit and a reporting unit; the reading unit reads the signal to noise ratio of subcarriers from the receiver of a multi-carrier optical communication system at predetermined monitoring time intervals; the judgment unit judges whether a first predetermined number of subcarriers of which the signal to noise ratio is smaller than a first threshold value exist; the detection unit monitors the damage change of the system when the judgment unit judges that a first predetermined number of subcarriers of which the signal to noise ratio is smaller than the first threshold value exist, and determines a reason for the degradation of the signal to noise ratio according to the damage change of the system; and the reporting unit reports the degradation of the signal to noise ratio and/or the reason for the degradation of the signal to noise ratio. With the on-line fault detection device, method and system of the invention adopted, the change of various kinds of damage of the system can be monitored online, and change that threats the normal work of the system is warned against, and therefore, targeted pre-adjustment can be realized possibly.