According to the characteristics of optical communication, a low-complexity optical space time trellis code based on pulse position modulation(PPM) is proposed. The scheme is based on the idea of delay diversity of the transmitter, and establishes the correlation of signals in time and space. The feedback interface cancellation algorithm (FICA) is used at the receiving end, and the maximum likelihood decision is used to decode the eliminated interference signal. The proposed scheme reduces the complexity of decoding by sacrificing the performance of transmitting diversity, which reduces the computational requirement of the system. The computational complexities of the algorithm and the Viterbi decoding algorithm are analyzed with the antenna numbers of 2 and 3. The simulation results show that when diversity gain is the same，the decoding complexities of the proposed scheme are reduced by 93.75% and 95.84% respectively, compared with the Viterbi decoding algorithm, and the deteriorations of the bit error rate performance are only 3 dB and 4.77 dB.