Leaf senescence is an important factor that affects crop yield traits and is regulated by various factors. Here, we propose the photo-carbon imbalance hypothesis to explain the mechanism of rice leaf senescence. The main idea of this hypothesis is that carbon assimilation decreases faster than the absorption of light energy in photosynthesis during the late stages of rice growth, which ultimately results in leaf senescence. Our results indicate that endogenous ascorbic acid (Asc) plays an important role in leaf senescence by affecting the expression of senescence genes, thereby influencing photosynthetic capacity and consequently grain yield. The effects of exogenous Asc and methyl jasmonate (MeJA) on photosynthetic capability implied that the balance between photoreaction and carbon assimilation is regulated by exogenous antioxidants or accelerators of senescence. The results of the shading treatments indicated that shading will mitigate the photo-carbon imbalance and improve photosynthetic capacity, resulting in increased yields. Increasing antioxidant concentrations can enhance the reactive oxygen species (ROS) scavenging capacity, whereas shading reduces excess light energy, which may help to restore the photo-carbon balance.