When a laser pulse propagates through a cloud, the phenomena such as energy decay, angle diffusion, temporal broadening and spatial dispersion may occur for it. The Monte Carlo method is used to study the effect of cloud altitude and thickness on laser downward transmission. The energy, angular, temporal and spatial distribution values of a laser pulse after it transmits downward to the sea level through a cloud are simulated respectively in both a situation that the cloud thickness is fixed while the cloud altitude is changing and a situation that the cloud altitude is fixed while the cloud thickness is changing. The result shows that when the cloud thickness is fixed, the Full Width Half Maximum (FWHM) and spot size of the laser pulse at sea level increase obviously with the increase of the cloud altitude. However, its energy and angular distribution values remain unchanged. When the cloud thickness is increasing while the cloud altitude is fixed, the energy of the laser pulse at sea level decreases, its angular distribution firstly diffuses and then trends to be stable, its FWHM increases and its spot size firstly increases and then trends to be stable. This research is of a referential value to the performance evaluation of optical communication systems.