Satellite optical communication system model and its channel MPPM (multi-pulse position modulation),an advanced form based on pulse position modulation,has become a high-performance optical communications satellite modulation. Far-field laser pulse broadening mechanism and effects of clock jitter on the system are studied with the existence of background radiation,atmospheric turbulence and thermal noise of satellite optical communication. Gray-coded MPPM is provided to decrease the clock jitter and inter-symbol interference influence and Gray-code mapping method is designed by constellation diagram. (7,2) MPPM transmission efficiency is more than two times that of 16PPM. Word error probability and bit error probability are analyzed with Gray coded (7,2) MPPM of Poisson-channel. Simulation results show that the use of Gray coded MPPM can effectively resist the timing error and reduce the system bit error probability.