Abstract: The light reactions of photosystem Ⅱ (PSⅡ) is greatly associated with the photosynthetic capacity. In order to capture more detailed information describing the status of PSⅡ during leaf senescence and rapidly screen cotton (Gossypium L.) genotypes with different duration of photosynthetic capacity, the PSⅡ photochemistry of the first leaves counted from the stem top of three cotton genotypes ('Baimian1' 'Baimian5' and 'DP99B') presented different leaf senescence progresses in production were examined by chlorophyll a fluorescence (Chl F) analysis during leaf senescence. The results showed that 'Baimian1' 'Baimian5' and 'DP99B' were late, intermediate and early aging types, respectively, based on the performance index of light absorption (PI_ABS). The three genotypes complied with the similar patterns in electrical transferring inhibition accompanying leaf senescence. The depletion of oxygen-evolving complex (OEC) was obvious at the late growth stage. The inhibition of the acceptor side of PSⅡ was greater than that of the donor side. The electron flow that through the light reactions of photosystem Ⅱ and photosystem Ⅰwas significantly limited accompanying leaf senescence. With the duration of leaf senescence, the energy distributed to thermal dissipation and the primary quinone electron acceptors of PSⅡ (Q_A) restoration increased, and correspondingly the energy used to transport an electron into the electron transport chain beyond Q_A^- (the reduction state of Q_A) declined. However, three cotton genotypes showed greater and greater electron transferring inhibition, except the number of Q_A reduction events between time=0 and time to reach maximal fluorescence, in the order of 'DP99B' > 'Baimian5' > 'Baimian1' with the duration of leaf senescence. It can be seen that the chlorophyll fluorescence characteristics can quickly and noninvasively reflect the senescence and the internal physiological mechanism of leaf senescence among different cotton genotypes.