Abstract: In the large scale MONET(metro optical network) construction, one of the critical issues may crash PON network stability is, software or hardware failures resulting in abnormal light emitting ONU, which impact the quality of services carried by PON severely. Therefore, it’s important to find out an effective approach to detect the abnormal light emitting ONU and get rid of it. This is significant to the development of FTTH as well. This paper described a creative fault detection mechanism based on the thorough analysis to root causes of abnormal light emitting ONU.