Abstract: 【Objective】 To study the differences in agronomic traits, seed cotton yield and fiber quality of different island cotton varieties under different nitrogen fertilizer levels in the hope of providing a theoretical basis for the future selection of high-yield and high-quality island cotton varieties and reasonable nitrogen application rates. 【Methods】 In the experiment, 12 sea island cotton varieties bred in the past two decades were used as test materials, and three nitrogen application treatments were set N₀ (0 kg/hm²), N₂₄₀ (240 kg/hm²), N₄₈₀ (480 kg/hm²), to study nitrogen application rates effects on the agronomic characteristics, fiber quality, yield composition and seed cotton yield of different island cotton varieties. 【Results】 The results of the study showed that nitrogen application could significantly increase the number of leaves, fruit branches and buds and bolls of island cotton, and the cotton yield by the nitrogen application treatment was higher than that of the non-nitrogen treatment; Among the cultivars, Xinhai 28 and Miancheng 10 performed better in comprehensive agronomic traits, and could maintain a relatively low dropout rate of bolls under different nitrogen fertilizer levels, thus ensuring the number of bolls.Nitrogen fertilizer had a significant effect on the number of bolls per plant and the yield of seed cotton, which was N₂₄₀>N₄₈₀>N₀, and had no significant effect on other yield traits; among varieties, the number of bolls per plant, boll weight per plant and seed cotton yield were all significant.The difference between the number of bolls per plant and the weight of bolls per plant under different nitrogen levels for Miancheng 10 and Xinhai 28 resulted in higher seed cotton yield.In terms of fiber quality, nitrogen fertilizer only had a significant effect on the average length of the upper half, showing a downward trend with the increase of nitrogen application rate, and had no significant effect on other indicators.However, there were extremely significant differences between varieties, indicating that the fiber quality was mainly affected by genetic characteristics, the fiber length, uniformity, and breaking strength of the upper half of Xinhai 28 and Xinhai 45 performed well under different nitrogen fertilizer levels, and the micronaire value reached the A-level standard. 【Conclusion】 In summary, Miancheng 10 and Xinhai 28 have excellent agronomic traits under different nitrogen fertilizer levels, and their yield traits are relatively coordinated.Therefore, higher seed cotton yield and fiber quality can be achieved under different nitrogen fertilizer levels.Regression analysis speculates that when the nitrogen application rate is 332.2 kg/hm², the seed cotton yield is the highest, which is 5025.1 kg/hm².