The present invention discloses a method for ranging in a passive optical network, and the method includes: obtaining a Round Trip Delay (RTD) between an Optical Line Terminal (OLT) and an Optical Network Unit (ONU); and opening a quiet window used for the ranging for the ONU according to the RTD to perform the ranging on this ONU. The present invention further discloses an apparatus for ranging in a passive optical network, and the apparatus includes: an obtainment module which is configured to: obtain a RTD between an OLT and an ONU; and a ranging module configured to: open a quiet window used for ranging for the ONU to perform ranging on the ONU according to the RTD. The present invention shortens the open time of the quiet window used for the ranging and improves the efficiency of the upstream transmission, and the implementation method is simple and convenient.