Embodiments of the present invention disclose a method and an apparatus for detecting an ONU, and a passive optical network system. The method includes detecting an identity code of an ONU in an open uplink empty window or an empty timeslot, and determining that an ONU corresponding to the identity code of the ONU is a rogue ONU according to the identity code of the ONU. A corresponding apparatus and passive optical network system are also provided in the embodiments of the present invention. In the passive optical network system, a rogue ONU is detected and determined quickly and efficiently, and an effect on an uplink service is reduced.