An optical signal transmission system, comprising: a first coherent transceiver, an optical transport unit, and N second coherent transceivers, wherein the first coherent transceiver is configured to convert N channels of downlink data into N modulating signals, wherein the N modulating signals each have a different frequency, each of the N modulating signals comprises a first sub-signal in a first polarization state and a second sub-signal in a second polarization state, the first polarization state is orthogonal to the second polarization state, and the first sub-signal and the second sub-signal each carry all information comprised in the downlink data corresponding to the respective modulating signal; the first coherent transceiver is further configured to: convert the N modulating signals into a first wavelength division multiplexing signal, and send the first wavelength division multiplexing signal to the optical transport unit, wherein the first wavelength division multiplexing signal comprises N first optical signals, and the N first optical signals correspond to the N modulating signals; the optical transport unit is configured to: receive the first wavelength division multiplexing signal sent by the first coherent transceiver, convert the first wavelength division multiplexing signal into N second optical signals through transmission processing, and send the N second optical signals to the N second coherent transceivers; and the N second coherent transceivers are configured to: each respectively receive the corresponding one of the N second optical signals sent by the optical transport unit, and process the N second optical signals to obtain information in downlink data carried in the N second optical signals, wherein the i th second coherent transceiver is configured to process the i th second optical signal to obtain information in downlink data carried in the i th second optical signal, and i is an integer greater than or equal to 1 and less than or equal to N. the first coherent transceiver is configured to convert N channels of downlink data into N modulating signals, wherein the N modulating signals each have a different frequency, each of the N modulating signals comprises a first sub-signal in a first polarization state and a second sub-signal in a second polarization state, the first polarization state is orthogonal to the second polarization state, and the first sub-signal and the second sub-signal each carry all information comprised in the downlink data corresponding to the respective modulating signal; the first coherent transceiver is further configured to: convert the N modulating signals into a first wavelength division multiplexing signal, and send the first wavelength division multiplexing signal to the optical transport unit, wherein the first wavelength division multiplexing signal comprises N first optical signals, and the N first optical signals correspond to the N modulating signals; the optical transport unit is configured to: receive the first wavelength division multiplexing signal sent by the first coherent transceiver, convert the first wavelength division multiplexing signal into N second optical signals through transmission processing, and send the N second optical signals to the N second coherent transceivers; and the N second coherent transceivers are configured to: each respectively receive the corresponding one of the N second optical signals sent by the optical transport unit, and process the N second optical signals to obtain information in downlink data carried in the N second optical signals, wherein the i th second coherent transceiver is configured to process the i th second optical signal to obtain information in downlink data carried in the i th second optical signal, and i is an integer greater than or equal to 1 and less than or equal to N.