Embodiments of the present invention provide an optical network unit ONU registration method, an apparatus, and a system. The ONU registration method includes: scanning, by an ONU port, a downstream wavelength channel, where the ONU port is one of a plurality of ports on an ONU; receiving, by the ONU port, a discovery grant message from an optical line terminal OLT, where the discovery grant message includes a channel identifier of a downstream wavelength channel on which the discovery grant message is being transmitted; sending, by the ONU port, a register request message to the OLT, where the register request message includes a port number of the ONU port; and receiving, by the ONU port, a register message from the OLT, where the register message includes the port number of the ONU port and an ONU logical identifier allocated by the OLT to the ONU port. In the embodiments of the present invention, the ONU port receives a discovery grant from the OLT, sends a register request to the OLT, and receives, from the OLT, the ONU logical identifier allocated to the ONU port. This provides an efficient ONU registration mechanism, to perform ONU management and maintenance at a granularity of an ONU port, so that the ONU can support more bandwidth channels, and a bandwidth is increased.