Abstract: There remains a lack of solid evidence revealing how radiation composition affects understory radiation transfer and gross primary productivity (GPP) across different forest layers. In this study, we used carbon flux and microclimate data from different forest layers to estimate the proportion of diffuse radiation (DF) via the Boland-Ridley-Lauret (BRL) model in a Pinus armandi plantation in central Yunnan. We then compared the distribution and daily variation of DF between the growing and non-growing periods. Standardized regression coefficients were computed to quantify the contributions of direct and diffuse components to understory photosynthetically active radiation (PAR). Finally, we applied the Michaelis-Menten equation to fit the light response to direct and diffuse radiation in both the canopy and understory, aiming to explore the photosynthetic mechanisms within different forest layers. The results showed significant seasonal differences in DF, with high and stable values occurring during the growing period. The contributions of direct and diffuse radiation to understory PAR also exhibited seasonal variations. Direct radiation dominated during the non-growing period (β^*_dir = 0.712 > β^*_dir = 0.370), whereas diffuse radiation dominated during the growing period (β^*_dir = 0.641 > β^*_dir = 0.468). Furthermore, the photosynthetic responses of different forest layers demonstrated hierarchical complementarity. The upper canopy exhibited a higher maximum photosynthetic capacity under direct radiation conditions (P_max = 0.925), while the understory demonstrated higher initial quantum efficiency under diffuse radiation conditions (α = 0.043). In summary, the higher and more stable DF during the growing period of the Pinus armandi plantation contributes to increased diffuse radiation to the understory, thereby enhancing understory light use efficiency. This complements the high assimilation capacity of the canopy under direct radiation conditions, ultimately enhancing the overall GPP of the plantation at the community scale.