The characteristics of laser pulse transmitting through atmosphere to oceanic channel is analyzed in detail. The method of Monte Carlo and theoretical analysis are adopted for modeling the laser communication channel. The method and procedure for simulation are introduced. The beam projected area spread of laser pulses transmitting through different thickness of cloud, and the pulse-energy distributions in spatial and temporal domain through different depth of sea water, and the broadening mechanism and channel noise are also studied .The conclusion is that broadening effect by cloud is saturated when the cloud thickness exceeds 500 m, and the random fluctuation of signal energy is increased for deeper water, but the full width at half width of energy distribution is not increased. The overall atmospheric and oceanic channel can be described by time-delay filter model.