Abstract: 【Objective】 To clarify the effects of defoliants on the physiological characteristics of cotton varieties with different sensitivities and whether the seedling stage can be used as a period for screening defoliant-sensitive varieties. 【Methods】 Two varieties with different sensitivities (Shida 5203, sensitive; 153, insensitive) to defoliants were selected as materials to compare the physiological characteristics between sensitive and insensitive varieties after spraying defoliants at both seedling and boll opening stages. 【Results】 The results showed that after defoliation treatment at a concentration of 10mg/L at the seedling stage, the net photosynthetic rate (Pn), stomatal conductance (Gs), transpiration rate (Tr), chlorophyll a, total chlorophyll, and auxin (IAA) contents of Shida 5203 and 153 leaves were significantly reduced compared to the control (water treatment), while the content of malondialdehyde (MDA) and abscisic acid (ABA), superoxide dismutase (SOD) and peroxidase (POD) activity were significantly increased compared to the control, defoliation treatment caused damage to cotton leaves, disruption of the photosynthetic system and hormone balance. Compared with the insensitive variety 153, the sensitive variety Shida 5203 showed a significant decrease in Pn, Gs, Tr, chlorophyll a content, total chlorophyll content and IAA content, a significant increase in MDA and ABA content and a small increase in SOD and POD enzyme activity, indicating that the sensitive variety Shida 5203 suffered greater damage on its leaves, producing fewer protective enzymes SOD and POD to resist external invasion, and less IAA and more ABA promoting leaf abscission. After spraying defoliants at the seedling stage, the trend of changes in various physiological indicators was consistent with that at the boll opening stage, and the content of MDA, IAA and ABA were significantly different between the two varieties after the spraying defoliants at both seedling and boll opening stages. 【Conclusion】 The seedling stage can be used as a period for screening defoliant-sensitive varieties, and the content of MDA, IAA and ABA after spraying defoliant can be used as physiological indicators for screening defoliant-sensitive varieties.