The regulation of photosynthetic functions during leaf senescence may be associated with leaf lifespan, but how to accurately reveal photosynthetic regulation during senescence based on significant differences in the leaf lifespan remains unresolved. Accordingly, this study evaluated sorghum cultivars with markedly disparate leaf lifespans. WSC62 was characterized by an extended leaf lifespan and earlier initiation of senescence, which resulted in a slower rate of leaf senescence than WSC34. In leaves, having different senescence onset times influenced the comparison of photosynthesis during leaf senescence between the two cultivars. The two cultivars showed significant differences in photosynthetic rate and photosystem II activity based on their senescence initiation timing throughout the leaf lifespan. The senescence rate of WSC62 was slower than that of WSC34. However, differences in the photosynthetic function at corresponding senescence stages were significantly reduced in both cultivars after normalization by the leaf senescence duration. Notably, WSC62 exhibited a lower photosynthetic rate and a higher senescence rate than WSC34. This was further validated by data from four additional sorghum cultivars. Consequently, selecting the onset of senescence based on the leaf lifespan was conducive to revealing variations in photosynthetic function during leaf senescence among sorghum cultivars. Furthermore, normalization by the leaf senescence duration demonstrated that there were patterns related and unrelated to the synergistic interaction between lifespan and senescence in the photosynthetic regulation of senescing leaves. The relationship between the patterns of photosynthetic regulation during leaf senescence and cultivar improvement are also discussed.