Abstract: In order to select the suitable leaf removal agent for the time and the effect of leaf removal, the effects of different time of leaf and leaf removal agent on endogenous hormone content, antioxidant enzyme activity and malondialdehyde (MDA) content in cotton leaves under field conditions were studied, and the effects of the leaf removing agents on the physiological effects of cotton were addressed as well. Using the split plot experiment design, the main area of the time of the leaf removal agent was sprayed, and the types of the leaf removal agent were tested. The result showed that after spaying the treatment of 540 g/L dropp ultra (mainly components 18% thidiazuron and 36% diuron) and 5.5% thidiazuron suspending agent, the content of abscisic acid (ABA) and superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) activity in cotton leaves increased at first and then decreased, the content of MDA increased and indole-3-acetic acid (IAA) content decreased, in which the dropp ultra on membrane system severely damaged. Spaying the treatment of 540 g/L dropp ultra when the cotton boll opening rate reaches 50%, cotton defoliation rate and boll opening rate can reach 90.1% and 99.9% respectively after 25 days.