The present invention discloses a passive optical network communications method: reporting, by an optical network unit, ONU, a calibration record of the ONU, where the calibration record includes an ID of a calibrated wavelength channel; sending a first message to the ONU when the OLT determines, according to the calibration record, that a target wavelength channel ID corresponding to a target wavelength channel to which the ONU needs to switch is not in the calibration record, where the first message includes a forced wavelength switching flag; and instructing the ONU to switch to the calibrated target wavelength channel. In this way, the ONU can implement wavelength switching quickly after calibrating a new wavelength channel so as to perform data communication over the calibrated new wavelength channel.