T-PPM code combines trellis coded modulation (TCM) and pulse amplitude modulation (PPM). It has both advantages in error correction and power efficiency of emission signal, and it has broad application prospects in space optical communications. The traditional T-PPM modulation and coding has shortage in mapping steps which limits the reliability of the system. In order to improve the bit error performance of atmospheric turbulence optical communication system, an improved T-PPM technology suitable for singlecarrier weak turbulence is proposed. It uses Gray code mapping instead of the traditional“subset”mapping of TCM to reduce the possibility of error decision in receiving. Simulation results show that Gray code mapping can obviously improve bit error performance; at the bit error rate (BER) level of 10-3, compared with“subset” mapping, the Gray code mapping increases coding gain by about 0.2 dB; at pulse expansion level (1 ps and 2 ps), the Gray code mapping scheme can reduce the BER to about 1/2 and 4/5 of that of conventional “subset” mapping scheme, thus proving the priority of Gray code mapping.