A power saving method in a passive optical network, including: when receiving an upstream burst timeslot allocated by the Optical Line Terminal (OLT) after a power supply of the Optical Network Unit (ONU) is turned off, the ONU dividing the upstream burst timeslot into one or more upstream burst timeslots; and the ONU turning on own laser in the upstream burst timeslot belonging to the ONU, and after sending one or more emergent Physical Layer Operations, Administration and Maintenance (PLOAM) messages to OLT in each upstream burst timeslot, promptly turning off the own laser. A power saving system in a passive optical network and an optical network unit is provided. The invention can decrease the power supply volume of an electric supply installation in a power-off ONU and reduce the cost of the electric supply installation.