A signal transmission system for a peer-to-peer optical network. The system includes an optical line terminal, an optical distribution node, and a plurality of optical network units. The optical network unit and the optical distribution node are connected in a tree distribution having an ordered relation. The optical line terminal transmits optic signals via the optical distribution node to a first ordered optical network unit, to allow the first ordered optical network unit to process the optic signals and to generate combined optic signals, which are transmitted to a next ordered optical network unit via the optical distribution node. The above steps are iterated, until a last ordered optical network unit transmits combined optic signals to the optical line terminal via the optical distribution node.