The strong sky background radiation is the biggest difficulty for star detection during daytime, and NIR can detect sufficient number of stars for navigation during daytime. At present, star detection under the ideal situation has been analyzed during daytime. However, for the actual situation, sky background is much different as the ideal situation, especially clouds has thermal radiation and scatter the solar radiation during daytime. It has great effect on star detection;the radiation of clouds and sun is calculated through empirical formula, the influence of clouds on star detection is analyzed. Using the stratus and cumulus clouds as an example, the star radiation attenuation through the clouds is specifically analyzed. It is helpful to the practical work.