Space-time Block Code (STBC) can achieve full diversity gain, but it can not provide code gain. So the system error rate capability will be restricted. In this paper, the optical Multiple Input Multiple Output (MIMO) system channel model was introduced at first. Then, for Q-PPM (Q>2) modulation, the existing STBC is amended. Moreover, by concatenating RS code, a concatenated STBC is proposed and its upper bound of error rate is analyzed. At last, the feasibility of this method is proved by the simulation experiment result. Compared the result with repetition code that concatenated RS code, it is shown that in atmosphere laser communication, the concatenated STBC can provide higher code gain.