Abstract: As an important part of forest ecosystem, forest understory plays an important role in maintaining plant diversity and stability of forest ecosystem. However, the scale difference between canopy reflectance and forest background reflectance is not clear, which is very limited in the study of forest understory by using single-angle optical remote sensing data. In this study, the relationship between forest background reflectance and canopy reflectance under different forest structure was analyzed by using geometric optical model 4-scale. The results show that the scale difference between forest background reflectance and canopy reflectance varies with the forest structure, and cannot be eliminated by vegetation index calculation. There is a significant linear relationship between forest background reflectance and forest canopy reflectance. The linear relationship varies with the wavelength and forest structure. And the model parameters of this linear relationship are highly related with leaf area index (LAI). Coefficients of determination at 680 nm and 865 nm are 0.881, 0.834 3, 0.890 6 and 0.880 3, respectively. This work provides a reference for the study of weakening or eliminating the scale difference between forest canopy and background reflectances.