In the deep-space laser communication downlink, the pulse position modulation (PPM) laser signal will have a pulse broadening effect when it is transmitted through the atmospheric channel and received by the single-photon detector, and causes the performance of the communication system to decrease. This paper analyzes the pulse broadening effect caused by light cumulus cloud scattering, atmospheric turbulence and aerosol scattering in the atmospheric channel and the jitter characteristics of the single photon detector. On this basis, this paper analyzes the effect of the physical thickness of the pale cumulus cloud on the communication rate under different PPM modulation orders and the jitter loss caused by the pulse broadening of the single photon detector. To compensate the effect of pulse broadening, a method based on likelihood ratio demodulation is proposed which reduce the effect of pulse broadening on communication BER through simulation verification. This research has certain reference significance for analyzing and evaluating the link performance of deep space PPM laser communication system.