Embodiments of the present invention disclose an optical signal transmission system and an optical signal transmission method. A specific solution is as follows: a first coherent transceiver is configured to: convert N channels of downlink data into N modulating signals, convert the N modulating signals into a first wavelength division multiplexing signal, and send the first wavelength division multiplexing signal to an optical transport unit; the optical transport unit is configured to: receive the first wavelength division multiplexing signal, convert the first wavelength division multiplexing signal into N second optical signals, and correspondingly send the N second optical signals to N second coherent transceivers; and one of the N second coherent transceivers is configured to: receive the N second optical signals, and process the N second optical signals to obtain information in downlink data carried in the N second optical signals.