A method for detecting an optical network unit (ONU) in a passive optical network (PON), an ONU, a PON and an optical line terminal (OLT) are disclosed. In an embodiment the method include detecting an ONU identity code in an open uplink empty window or an empty timeslot, wherein the ONU identity code is a specific code stream sequence of the ONU that identifies a single ONU of the plurality of ONUs included in the PON system during the open uplink empty window or the empty timeslot and determining that the single ONU corresponding to the ONU identity code is a rogue ONU according to the ONU identity code.