Target-background contrast is one of the important parameters for indentification of target in space.Since cloud covers about 50% the ground surface and has important influence to the radiation transfer,it is necessary to taking into account the influence of cloud in the computing process of the target- background contrast. In the paper, the effect of the optical properties of cloud on the target-background contrast is analyzed based on the plane parallel assumption and the probability of cloud free light of sight (PCFLOS).