Due to the limitations of light dispersion and speed restriction of electronic devices, both the transfer rate and the performance of Free Space Optical (FSO) are limited. In order to sufficiently exploit the potentials of FSO to enhance the transfer rate and the bit error rate (BER) performance of the optical communication system, it needs to adopt a new transmission technology which owns high spectrum efficiency and high ability to combat the atmospheric scattering effect. Hence, the two mature technologies of OFDM and FSO are combined in this paper. The system model is built according to the basic principles of FSO and OFDM. Finally, the systems BER performance with different types of modulation, different numbers of carrier, and different guard interval lengths are simulated by Matlab.