Abstract: Free-space optical communication(FSO) system is a new type of broadband access systems, optical fiber communication can solve the bottleneck problem with the microwave communications, combines the advantages of optical communications and microwave communications, the FSO systems are vulnerable to the impact of environmental and weather conditions, the Multiple Input Multiple Output(MIMO) technology in FSO systems can effectively improve the atmospheric effects of the laser beam to improve the system performance. In this paper, space-time decoding problem of FSO-MIMO system was mainly studied. Combined with some of the traditional decoding algorithms, the iterative decoding Turbo codes were used in FSO MIMO idea of space-time decoding, the analysis based on Markov chain Monte Carlo (MCMC) algorithm for iterative decoding methods was also made.