Space-time trellis code(STTC) has the advantage of high spectrum efficiency. A STTC coding scheme was proposed for M-PSK modulated intensity modulation/direct detection(IM/DD) atmospheric optical communication system. By using the proposed mothed, the problem that conventional STTC is unable to be directly applied to IM/DD link was eliminated by employing the IFFT symmetry and a direct-current (DC) component. The system BER performance in different atmospheric scintillation and modulation index was evaluated by Monte Carlo method. Simulation results show that the proposed scheme has the advantage of mitigating the scintillation effect caused by atmosphere turbulence, and the link BER performance improves with the increase of diversity gain particularly in strong turbulence channels.