Embodiments of this application disclose a communication method based on a passive optical network, a related device, and a system. In a scenario in which the PON supports a plurality of wavelength paths, an OLT can flexibly indicate, to an ONU, an upstream wavelength path used to transmit upstream service data, to improve bandwidth utilization of the PON. The method includes: An OLT sends a first downstream frame to an activated ONU by using at least one downstream wavelength path, where the first downstream frame includes a first path identifier used to identify a first upstream wavelength path; and the OLT receives upstream service data from the activated ONU by using the first upstream wavelength path.