Quasi-orthogonal space-time block coding (QOSTBC) is a transformational structure of orthogonal space-time coding. It is a time and space diversity technique. It has a highly efficient diversity utilization and coding efficiency. Over atmospheric turbulence channels, QOSTBC is an optical communication solution. By the study of the atmospheric channel characteristics, a QOSTBC model based on spatial diversity optical communication is proposed. The principle of QOSTBC is introduced. The channel capacity and bit error rate (BER) are analyzed. Using Monte Carlo simulation method, the channel capacity and the BER between diversity and no diversity coding system are compared. The results show that by using of QOSTBC the system′s channel capacity increases with the number of transmit antennas. The error performance is improved effectively. The system signal noise ratio (SNR) reduces 9 dB comparing QOSTBC and Alamouti coding system, which indicates that the coding system has provided a well channel capacity and error performance. The coding system can effectively inhibit optical intensity fluctuation of optical communication affected by atomspheric turbulance.