Changes in diffuse radiation received at the ground surface impact light use efficiency (LUE) of the forest ecosystem. In this study, we focused on a subtropical plantation coniferous forest at Qianyanzhou. Based on 30 min flux data, routine meteorological data, and two indexes, clearness index (k_t) and the ratio (S_f/S₀) of diffuse radiation (S_f) to global solar radiation (S₀), we analyzed the impacts of changes in diffuse radiation on light use efficiency in this ecosystem during the mid-growing season (June to August) from 2003 to 2012. Moreover, applying the modified light response model, we investigated effects of changes in diffuse radiation on vegetation photosynthetic characteristics. The results indicated that the LUE decreased by a mean of 44.66% in k_t range of 0.6 to 0.7 during the mid-growing season from 2003 to 2012,compared that in k_t range of 0.4 to 0.5. In the same period, the LUE increased by a mean of 22.24% in S_f/S₀ range of 70% to 85%, compared to that in S_f/S₀ range of 55% to 70%. These results indicated that LUE was higher under cloudy skies and aerosol loading increase conditions than that under clear sky conditions. Moreover, the initial quantum efficiency (α_f) increased by a mean of 0.63 gCO₂/mol under diffuse radiation, comparing with that (α_r) under direct radiation in 2005, 2006, 2008, 2009, 2010, and 2012 during the mid-growing season without drought stress. Regardless of where the water stress occurred, the capacity of a canopy to resist photosynthetic saturation at high levels of PAR (β_f) was greater under diffuse radiation than β_r under direct radiation by a mean of 0.81 mgCO₂ m^-2 s^-1. The results verified that diffuse radiation improved the photosynthetic capacity of the forest ecosystem.