Abstract: 【 Objective 】 The defoliation and boll opening rate of cotton germplasm resources in southern Xinjiang were studied after spraying defoliant in order to screen out the germplasm which might be sensitive to defoliants and provide reference for selecting and breeding sensitive cultivars to defoliant. 【 Method 】The defoliant (thidiazuron) with good defoliation effect was used (combined with ethephon) in the project to separately investigate the number of leaves and boll opening rate at different times after spraying defoliant. 【 Result 】 The defoliation rate of different cotton varieties was significantly different on the 4th and 7th days. The varieties with defoliation rate more than 95% included Xinluzhong 2, Xinluzhong 5, and Hexin 27, etc; the post-drug boll opening rate varied among different varieties; the defoliation rate and defoliation effect of different types of leaves were different; 90 germplasm resources were classified into 4 categories by cluster analysis, and the second category was sensitive to defoliants. 【 Conclusion 】The defoliation rate and the boll opening rate of different varieties were different, and the defoliation rate at the 4th to 7th day could be used as the index to evaluate the sensitivity of upland cotton varieties to defoliant in southern Xinjiang. As to the effect of spraying defoliant, the fruit branch leaf was the best, followed by the foliage leaf, the main stem leaf was the worst. The cluster analysis showed that the second class was the most sensitive germplasm to defoliant.