In the context of climate change, the increasing drought events have important impacts on tree growth and carbon storage. However, studying on response mechanisms of the leaf photosynthetic carbon sequestration and biomass carbon storage of trees to the prolonged drought conditions is still limited. In this study, a 70-year-old natural deciduous oak (Quercus aliena var. acuteserrata) forest was selected to carry out a long-term throughfall exclusion (TFE) experiment to investigate the responses of photosynthetic carbon sequestration and carbon sink potential after long-term TFE treatment. In this study, the results showed that TFE treatment significantly reduced the net photosynthetic rate (A), maximum photochemical efficiency (V_cmax), maximum electron transport rate (J_max), and maximum quantum efficiency of PSII (F_v/F_m) in leaf of Q. aliena var. acuteserrata. And the correlation between A and g_s, J_max and F_v/F_m of Q. aliena var. acuteserrata was enhanced by the long-term TFE treatment. That is, the seven consecutive years of the TFE treatment significantly reduced the photosynthetic carbon sequestration capacity of Q. aliena var. acuteserrata. On the one hand, the Q. aliena var. acuteserrata alleviated and adapted the adverse TFE treatment effects by increasing specific leaf area (SLA), ratio of leaf palisade tissue to spongy tissue, and stomatal density. On the other hand, the Q. aliena var. acuteserrata reduced the leaf area index (LAI) and transpiration water loss after the TFE treatments, which could improve the water use efficiency (WUE) of Q. aliena var. acuteserrata under the long-term drought stress. However, our study found that the long-term TFE treatment still reduced the biomass carbon storage, and the biomass carbon of all organs in Q. aliena var. acuteserrata decreased significantly. In this study, the leaf biomass carbon decreased by 30.6%, stem biomass carbon decreased by 34.3%, underground biomass carbon decreased by 26.4%, and total biomass carbon decreased by 32.8% after 7 years of the TFE treatment. The significant increase in the ratio of belowground to aboveground biomass carbon indicated that Q. aliena var. acuteserrata adapted drought stress by increasing underground growth along with increasing carbon input. In the context of changing rainfall pattern in the future, the adaptive adjustment of the growth and leaf traits of oak forest is an important strategy for its long-term survival. The study provides a scientific basis for evaluating the changes of photosynthetic carbon sequestration capacity and carbon storage of warm temperate forest ecosystems under the drought stress induced by global climate change.