Embodiments of the present invention provide a logical link identifier (LLID) registration method, device, and system. The method includes: receiving, by a coax media converter (CMC), a first registration request sent by at least one coaxial network unit (CNU) that is connected to the CMC, where the first registration request carries the media access control (MAC) address of the CNU; and sending, by the CMC, in a first data window allocated to the CMC by an optical line terminal (OLT), a second registration request to the OLT, where the second registration request carries a first level LLID identifier allocated to the CMC by the OLT and the MAC address of the CNU, and the first data window is used for data exchange between the CMC and the OLT.