Abstract Downward surface shortwave radiation (DSSR) is important in various disciplines and is influenced by the atmosphere and mountainous topography. In this paper, an integrated approach that incorporates a mountainous spectral radiation scheme and an atmospheric broadband transmittance model based on a fine-resolution digital elevation model (DEM) and MODIS atmospheric products is proposed to estimate DSSR. This approach accurately considers important terrain and atmospheric factors and avoids any dependence on ground-based observations except for validation purposes. The results demonstrate that the radiation components exhibit very high spatial heterogeneity and are largely affected by different terrain orientations (especially topographic obstructions), slope, the cosine of the solar illumination angle, and atmospheric environments. Ground-based measurements for 56 clear-sky days from two observation stations are used for validation in the Dayekou watershed of the Heihe River Basin. The experiments show that this algorithm performs well; both the mean bias error percentage (MBE%; −6.2%) and the root-mean-square difference percentage (RMSD%; 7.5%) are less than 10%.