Abstract: 【Objective】 The technology of defoliation and ripening is an important prerequisite to realize cotton harvesting and reasonable application time can achieve good defoliation and ripening effect and reduce the influence on cotton yield and fiber quality. 【Method】 In this experiment, the effects of defoliation on cotton boll development and damage degree to single boll fiber and cottonseed were analyzed by setting different time of defoliation and ripening agent, and cotton boll stage suitable for defoliation and ripening was discussed. 【Result】 The boll period of cotton was shortened by applying defoliation and ripening agent. The shorter the boll period was, the more shortened boll period was. The weight of fiber per boll and cotton seed weight per boll was significantly affected by the defoliation agent, and the decrease of boll period was the least when the boll age was 44 days, which was reduced by about 0.15 g. 【Conclusion】 According to the relationship between boll stage and the damage amount of single boll factor, the application time of defoliation can be determined. If the damage amount of single boll yield was less than 0.1 g, the application time of defoliation agent / boll period should be greater than 0.79, and cotton varieties with 55 days boll stage could apply defoliation agent at 43 days of boll age.