Based on analyzing the restriction of space-time coding used in the free space optical communication, the intensity superposition code is propounded to increase the channel capacity of the optical multiple-input multiple-output (MIMO) system. The code/decode rules for the system are designed. The channel capacities of different system forms are analyzed. The decision thresholds are chosen, and the bit error rates (BERs) of different system forms are computed. The simulation result indicates that the channel capacity of this system is more than doubled channel capacity of the traditional system. And the BER falls very fast when the signal-to-noise ratio (SNR) is more than 10 dB.