An orthogonal space-time block code suitable for intensity modulation/direct detection optical communication system is constructed by using orthogonality of pulse position modulation (PPM) after analyzing the effect of atmosphere on the optical signal. The technique is applied to the first layer of the layered space-time code (LST) to increase the diversity of the information symbol. The effect of other layers on the symbol is eliminated by using group interference suppression (GIS). The signal is recovered by combining the decoding method of space-time block code (STBC) and Bell Labs Louyered Space-Time (BLAST) and maximum likelihood decision rule. The feasibility of the technique is proved by the simulation experiment. It shows that in free space optical communication the error rate performance of hybrid space-time code is better than BLAST.