Luby transform (LT) code is a rateless linear code, which has good channel adaptability and low complexity. A concatenated coding scheme of CRC code and LT code (CRC-LT code) was proposed to ensure the reliability of FSO system and improve the coding gain of LT code in FSO system by adjusting overhead. A new analytic model was developed to derive the data restoration probability with given conditions of channel. Finally, simulation experiments were performed over Gamma-Gamma channel model, and the relationships among data restoration probability, channel condition, SNR and overhead were given. The results show that CRC-LT code has better coding gain and can effectively enhance the reliability of the FSO system by increasing overhead.