Embodiments of the present disclosure provide a message transmission method, a relay device, and a message processor, so as to reduce maintenance costs. The method includes: receiving, by a relay device, a downlink message sent by a message processor, where the downlink message includes message content and an address of an ONU; obtaining, by the relay device according to the downlink message, an identifier of the ONU, corresponding to the address of the ONU; and sending, by the relay device, the message content to the ONU according to the identifier of the ONU.