The invention discloses a method for transmitting downlink messages in a photoelectric hybrid system and optical coaxial units. The method includes connecting the optical coaxial units OCU with a plurality of coaxial distribution networks CDN, creating mapping relation information of coaxial network units CNU and OCU ports; transmitting the messages to the coaxial distribution networks CDN with target coaxial network units CNU via the OCU ports corresponding to the target coaxial network units CNU according to the created mapping relation information of the coaxial network units CNU and the OCU ports when the optical coaxial units OCU receive the messages which are transmitted from optical link terminals OLT to the target coaxial network units CNU. The coaxial distribution networks CDN with the coaxial network units CNU are connected to the OCU ports of the optical coaxial units OCU. The method and the optical coaxial units have the advantages that after the optical coaxial units OCU receive the messages from the optical link terminals OLT, mapping relation information of LLID (logic link identification) and RFID (radiofrequency identification) of RF (radiofrequency) ports is queried, the messages can be transmitted to the corresponding coaxial network units CNU via the corresponding coaxial distribution networks CDN, messages of all the other coaxial distribution networks CDN which are irrelevant to OCU connection are abandoned, accordingly, unnecessary copying procedures can be omitted, and the downlink messages can be completely efficiently forwarded.