The channel of free space optical communication (FSO) has to experience varied channel effects such as dust,rain,fog etc. The system which simple combined FSO with orthogonal frequency devision muitiplexing (OFDM) is more complicated although OFDM can resist the influence of scatter. We proposed a new time frequency code to solve the shortcoming of the traditional FSO-OFDM system. We established the channel model according to the characters of atmospheric channel. And particular described the compilation method of time frequency code and analyzed the relation between the system′s bit error rate (BER) and the channel characteristic after introducing the FSO-OFDM system model. At last,this scheme is confirmed by Monte Carlo approach. It is shown that this method not only can simplify the system but also can improve the error rate performance of the system effectively.