Abstract: The present invention discloses an optical communication system, a sub-rate multiplexing/de-multiplexing apparatus and the method thereof such that the optical communication system could meet the demand of Metropolitan Area Network (MAN) communications using the existing optical fibre network. This optical communication system includes an optical transmitting module and an optical receiving module connected by optical fibres, wherein the optical transmitting module is used for converting the inputted electrical signals into optical signals and transferring the optical signals to the optical receiving module via optical fibres, and the optical receiving module is used for converting the received optical signals into electrical signals and outputting the electrical signals. In the optical signals transferred by optical fibres, the data transmission rate of at least one wavelength is about 5 Gb/s.