The understory light quality of natural forests has importantly ecological significance for the composition and regeneration of tree seedlings, shrubs and grass. However, the knowledge on the understory light quality is still lacking. In this study, taking natural forests in eastern Jilin Province as an example, by monitoring tree data and understory light quality data, the relationship between forest canopy structure and understory light quality at different spatial scales was analyzed based on the moving window method. The results showed that there were differences between the red light photon flux density (R) and the blue light photon flux density (B) under different forest types. Among them, the blue light photon flux density was the lowest under the mixed forest of Abies holophylla-Carpinus cordata-Acer ukurunduense mixed forest, while the mixed forest of Abies holophylla-Tilia amurensis-Abies nephrolepis mixed forest and Larix olgensis pure forest were the largest. As the scale increased, the tree basal area of natural forest was significantly positively correlated with the ratios of R/PFD (red light photon flux density ratio/photon flux density ratio) and B/PFD (blue light photon flux density ratio/photon flux density ratio) (P<0.05). With the increase of scale, the correlation coefficient increased gradually, reaching a peak at 35 m. On this basis, there was a significantly positive correlation (P<0.05) when it was extended 10 m to the south, east, and west, respectively. When analyzing the influence of the dominant tree species on the ratios of R/PFD and B/PFD under this scale, it was found that the ratios of R/PFD and B/PFD increased with the increase of the basal area of the coniferous forest. Compared with the broad-leaved forest, the canopy structure under most coniferous forests was significantly positively correlated with the ratios of R/PFD and B/PFD under forest (P<0.05). Under different tree species, the tree canopy structure had different effects on R/PFD and B/PFD ratios. In general, differences in canopy structure in scale and tree species composition can affect the distribution of R/PFD and B/PFD ratios under forest to a certain extent. This study would help us understand the influence mechanism of canopy structure on the distribution of understory light quality, so as to provide a theoretical basis for natural forest regeneration, succession, restoration and rational management.