In order to overcome the influence of atmospheric turbulence on the free space optical communication (FSO) link, LT code is applied to FSO system. M-ary LT codes have higher transmission efficiency. A new degree distribution function is proposed to improve the decoding performance of M-ary LT codes. The decoding failure rate of LT codes with different base is calculated. The strong turbulence channel modeling is carried out and the performance of FSO with different coding schemes is simulated. The results show that the LT code with new degree distribution can reduce the failure rate of decoding, and can effectively improve the performance of FSO system in strong turbulence.