A system, a method and a related device for signal transmission are provided in order to improve utilization efficiency of the fiber. The system includes an optical line terminal (OLT), a plurality of optical network units (ONU), a base band unit (BBU), a plurality of remote radio units (RRU), a first signal-processing device and a plurality of second signal-processing devices, wherein the first signal-processing device sends a multiplexed signal to a plurality of second signal-processing devices after a plurality of signals sent by the OLT and the BBU are multiplexed, and demultiplexes a signal sent by the second signal-processing device in order to obtain a plurality of signals sent by the ONU and a plurality of different RRUs and sends these demultiplexed signals to the OLT and the BBU; one of a plurality of second signal processing devices sends a multiplexed signal to the first signal-processing device after a plurality of signals sent by the ONU and a plurality of different RRUs are multiplexed, and demultiplexes a signal sent by the first signal-processing device in order to obtain a signal sent by the OLT and a plurality of signals sent to a plurality of different RRUs by the BBU and sends these demultiplexed signals to the ONU and the RRUs.