Laser is always affected by atmospheric turbulence in atmosphere transmission, and channel estimation will become more difficult at poor atmospheric condition. According to Chadi Abou-Rjeily′s coding idea, introducing permutation matrix instead of negative form of symbols in the real area Alamouti method, utilizing binary pulse position modulation, a differential space-time code scheme that does not require channel estimates for free space optical communication is proposed. The simulation results show that, compared with Alamouti coding which requires channel estimates, this method can obtain the same diversity gain. At the same time, it is relatively simple and more feasible in practice.