The present invention discloses a passive optical network communications method, apparatus and system. The method includes: receiving, by an optical network unit, a first message sent by an optical line terminal, where the first message carries backup wavelength channel ID information; switching, by the optical network unit when the optical network unit detects a fault, an operating wavelength channel of the optical network unit to a backup wavelength channel identified by the backup wavelength channel ID information; and performing, by the optical network unit, data communication over the switched-to backup wavelength channel. In this way, fast protection switching of a passive optical network system is implemented and reliability of the system is improved.