Embodiments of the present invention provide an optical network unit ONU registration method, apparatus, and system, to resolve a problem in the prior art that a registration process is cumbersome. The method includes: receiving an uplink optical signal, where the uplink optical signal carries ONU authentication information; sending the optical signal to a corresponding MAC module separately according to a wavelength of uplink light; extracting, by the MAC module, the ONU authentication information, and sending the extracted ONU authentication information to a processor; and receiving, by the processor, the ONU authentication information, and determining whether the ONU authentication information is consistent with ONU authentication information configured by an OLT, where if the ONU authentication information is consistent with the ONU authentication information configured by the OLT, an ONU is registered successfully.