Abstract: 【Objective】 The aim of the study is to clarify the effects of defoliants on the development of cotton bolls and fiber quality at different development time of cotton varieties with different maturities in the hope of providing theoretical basis for the rational selection of defoliants suitable for the varieties deployed in cotton production. 【Methods】 Field experiments were carried out in 2020 and 2021. Two cotton varieties(early maturing products Shidamian 268 and Late maturing varieties Shidamian 451) that were of early or late maturity were used as the experimental materials. They were sprayed with two kinds of defoliants with the active ingredient being Ruituolong or Yeluokong. Spraying was applied on cotton plants with cotton bolls that had developed for 30, 35, 40, 45, 50 d. Three weeks after spraying, cotton bolls were harvested from each treatment, measured for single boll weight, lint percentage, fiber length, fiber uniformity, micronaire value, fiber strength and elongation rate, and then compared with CK. 【Results】 (1) Defoliants had little effect on boll weight when they were applied on early-maturing and late-maturing cotton variety with 40 and 50 d old bolls, respectively, but compared to Ruituolong, Yeluokong had a greater impact on boll weight. Both defoliants had significantly negative effect on lint percentage of cotton bolls that were less than 40 d old when defoliants were applied, but such negative effect on lint percentage was not significant for >45 d cotton bolls. (2) For the early-maturing variety, Ruituolong had no significant effect on micronaire of >45 d bolls, fiber elongation rate of >40 d bolls, fiber length and uniformity index of >35 d bolls, and fiber strength of >30 d bolls. For the late maturing variety, the unaffected traits were observed for fiber from bolls that were 5-10 d older compared to the early-maturing variety. Compared to Ruituolong, Yeluokong had a stronger impact on fiber quality, particularly on fiber of the late-maturing variety. 【Conclusion】 The effect of defoliants on yield components and fiber quality is minimized for >40 d bolls of early-maturing precocious variety Shidamian 268 and >45 d bolls of late-maturing Strain Shidamian 451 variety. Compared with Yeluokong, Ruituolong with less Diuron has weaker effect on cotton boll weight and fiber quality. Choose early-maturing varieties and spray defoliants with a low concentration of Diuron when bolls have developed for more than 40 d.