Provided are a method for managing an ONU in a passive optical network, an OLT and a system. The method includes determining a first quiet window and a second quiet window, and allocating the first quiet window to an ONU within a first preset distance range and allocating the second quiet window to an ONU within a second preset distance range, where the distance between the optical line terminal (OLT) and the ONU within the first preset distance range is less than the distance between the OLT and the ONU within the second preset distance range.