An apparatus for detecting online failure, applicable to a discrete multi-tone optical communication system, comprising: a reading unit configured to read signal to noise ratios of subcarriers from a receiver of the discrete multi-tone optical communication system according to a predetermined monitoring time interval; and characterised in that the apparatus for detecting online failure comprises: a judging unit configured to judge whether there exist a first predetermined number of subcarriers of which the signal to noise ratios are less than a first threshold value; a detecting unit configured to monitor a change of distortion of the system when it is judged yes by the judging unit, and determine a cause of degradation of signal to noise ratios according to the change of distortion of the system; and a reporting unit configured to report degradation of signal to noise ratios and/or the cause of degradation of signal to noise ratios, wherein the detecting unit is configured to one of: monitor channel gains of the system when it is judged yes by the judging unit, and determine whether the cause of degradation of signal to noise ratios comprises a change of the channel gains according to a monitoring result, and monitor a discrete multi-tone symbol synchronization error of the system when it is judged yes by the judging unit, and determine whether the cause of degradation of signal to noise ratios comprises the discrete multi-tone symbol synchronization error according to a monitoring result, and monitor a clock jitter of the system when it is judged yes by the judging unit, and determine whether the cause of degradation of signal to noise ratios comprises the clock jitter according to a monitoring result. a reading unit configured to read signal to noise ratios of subcarriers from a receiver of the discrete multi-tone optical communication system according to a predetermined monitoring time interval; and characterised in that the apparatus for detecting online failure comprises: a judging unit configured to judge whether there exist a first predetermined number of subcarriers of which the signal to noise ratios are less than a first threshold value; a detecting unit configured to monitor a change of distortion of the system when it is judged yes by the judging unit, and determine a cause of degradation of signal to noise ratios according to the change of distortion of the system; and a reporting unit configured to report degradation of signal to noise ratios and/or the cause of degradation of signal to noise ratios, wherein the detecting unit is configured to one of: monitor channel gains of the system when it is judged yes by the judging unit, and determine whether the cause of degradation of signal to noise ratios comprises a change of the channel gains according to a monitoring result, and monitor a discrete multi-tone symbol synchronization error of the system when it is judged yes by the judging unit, and determine whether the cause of degradation of signal to noise ratios comprises the discrete multi-tone symbol synchronization error according to a monitoring result, and monitor a clock jitter of the system when it is judged yes by the judging unit, and determine whether the cause of degradation of signal to noise ratios comprises the clock jitter according to a monitoring result. a judging unit configured to judge whether there exist a first predetermined number of subcarriers of which the signal to noise ratios are less than a first threshold value; a detecting unit configured to monitor a change of distortion of the system when it is judged yes by the judging unit, and determine a cause of degradation of signal to noise ratios according to the change of distortion of the system; and a reporting unit configured to report degradation of signal to noise ratios and/or the cause of degradation of signal to noise ratios, wherein the detecting unit is configured to one of: monitor channel gains of the system when it is judged yes by the judging unit, and determine whether the cause of degradation of signal to noise ratios comprises a change of the channel gains according to a monitoring result, and monitor a discrete multi-tone symbol synchronization error of the system when it is judged yes by the judging unit, and determine whether the cause of degradation of signal to noise ratios comprises the discrete multi-tone symbol synchronization error according to a monitoring result, and monitor a clock jitter of the system when it is judged yes by the judging unit, and determine whether the cause of degradation of signal to noise ratios comprises the clock jitter according to a monitoring result. monitor channel gains of the system when it is judged yes by the judging unit, and determine whether the cause of degradation of signal to noise ratios comprises a change of the channel gains according to a monitoring result, and monitor a discrete multi-tone symbol synchronization error of the system when it is judged yes by the judging unit, and determine whether the cause of degradation of signal to noise ratios comprises the discrete multi-tone symbol synchronization error according to a monitoring result, and monitor a clock jitter of the system when it is judged yes by the judging unit, and determine whether the cause of degradation of signal to noise ratios comprises the clock jitter according to a monitoring result.