For the echo signals of Satellite laser altimeter, the leading source of noise is the background radiation noise, which mainly comprises the reflection of sunlight by the targets and the atmospheric scattering of sunlight. This paper deals with a theoretical study on the reflection and atmospheric scattering of sunlight. And on the basis, we calculate and simulate the irradiance of extraterrestrial solar spectrum and the respective effects of path zenith angel, solar zenith angle and ground diffuse reflectance on the solar atmospheric radiance, concluding the corresponding relationship between background radiation noise and these parameters, which provides a theoretical basis for the improvement of Satellite laser altimeter.