Scale is a key issue in the spatial and environmental sciences. In this paper, we retrieved the sun-view geometries of each slope element of Badain Jaran Desert using ASTER GDEM data. Combined with the 30m resolution of Landsat ETM+ data, the characteristic of directional reflection of land surface reflectance was discussed. Then we derived a simplified physical BRDF model assuming that the main radiation source of land surface is direct solar radiation, and calculated the land surface reflectance of spatial resolutions from 30m to 500m and 1000m, which is compared with the results calculated by MODIS BRDF products in given sun-view geometries. Linear-regression analyses were performed to simulate and analyze scale characteristics. Moreover, some factors, such as solar zenith angle and azimuth angle, are taking into account for its fitting precision. The results show that our model is more precise in visible band than in nearinfrared and short-wave infrared bands, in which the sky diffuse radiation and adjacent terrain radiation cannot be ignored.