The combination of Faster-than-Nyquist (FTN) theory with modulation technology can effectively improve the spectral efficiency of atmospheric communication systems. In this paper, the FTN theory was introduced to atmospheric optical communications, and an FTN optical transmission system suitable for log-normal turbulence channel was proposed. Moreover, the equation for the Bit Error Rate (BER) of the proposed system was derived and further analyzed using the MonteCarlo method. The results indicate that the FTN scheme can considerably improve the spectral efficiency of atmospheric optical transmission systems. For example, when the signal-to-noise ratio is 18 dB, and the S.I. is 0.4, the spectral efficiency can reach 1.7 Baud/Hz, whereas it is only 1.56 Baud/Hz without the FTN scheme. Regarding the effects of atmospheric turbulence on system BER performance, when the S.I. is 0.4 and BER is 3.8×10-3, the error performance decreases by approximately 1 dB. Compared with the significant improvement in spectral efficiency, the decrease in BER performance is worthwhile.Therefore, FTN technology can be introduced into atmospheric optical transmission systems to improve spectral efficiency.