An optical signal transmission system and method of allocating center frequencies of intermediate frequency (IF) carriers in a frequency division multiplexing (FDM) optical fiber link. The optical signal transmission method includes determining a center frequency interval between modulated signals based on a bandwidth of the modulated signals or a center frequency of a modulated signal having a lowest center frequency, among the modulated signals, reallocating center frequencies to the modulated signals based on the center frequency interval between the modulated signals, and converting the modulated signals reallocated the center frequencies from electrical signal into optical signal and transmitting the optical signal.