For the free space optics (FSO) communication system, the atmospheric channel′s signal-noise-rate (SNR) is a key factor, and the FSO system based on twofold turbo code is proposed. First, the atmospheric influence on FSO system and the character of atmospheric channel are analyzed, the SNR of atmospheric channel versus the weather visibility is deduced, and the coding and decoding principles of twofold turbo code are analyzed. Applied the function of atmospheric channel′s SNR, the SNR of atmospheric channel versus the weather visibility and the bit error rate (BER) of FSO system based on twofold turbo code and common turbo code are simulated. The results show that, for an applied optical wireless communication system, under the condition of system′s BER being below 10-5, the channel′s SNR of system based on twofold turbo code is 1dB lower than that based on common turbo code. So, for the same channel′s SNR, the twofold turbo code is more efficient to degrade the system′s BER than common turbo code.