Abstract: 【 Objective 】To study and compare the dynamic changes of boll components in the development of cotton bolls and to explore the dynamic development law of Gossypium barbadense L. and Gossypium hirsutum L. in southern Xinjiang in the hope of providing scientific theoretical basis for the management of high-yield and high-quality cultivation and high-yield and high-quality breeding of cotton.【 Methods 】The bolls of Xinhai No.25 and Xinluzhong No.37 were taken as the research objects, and the external traits of boll development and dry weight of boll components were compared, and besides, logistic fitting was also carried out for comparison. 【 Results 】External traits: the length, diameter and volume of the bolls reached their maximum values 9, 9 and 15 d later than those of land cotton, and were 15.539 mm, 7.167 mm and 3.4 cm³ less, respectively; the length-to-diameter ratio of island cotton was 0.725 more than that of land cotton; the bolls of island cotton had 3-4 chambers and 20 seeds; the chambers of land cotton had 4-5 chambers and 36 seeds. Boll traits: the dry weight of the boll shells, seeds and fibres of sea island cotton reached its maximum value 12, 9 and 3 d later than that of terrestrial cotton, and 0.419, 0.654 and 1.204 g lighter; the ratio of boll shells∶seeds∶fibres was 29∶46∶25 for sea island cotton, and 24∶43∶33 for terrestrial cotton; the dry matter of each component of the bolls of sea island and terrestrial cotton was fitted with the Logistic equation, and both showed the same dry matter as that of the land. Fitting results were "S" curve, island cotton and land cotton boll fresh boll weight, boll dry weight, volume with boll age change trends were all similar; Island cotton boll length, boll diameter and cotton seed dry weight entered and ended the vigorous growth time 1-4 d later than terrestrial cotton.【 Conclusion 】The single boll weight, boll chamber and cotton seed number of upland cotton are higher than that of island cotton, the boll period is shorter than that of island cotton, and the proportion of boll shell and cottonseed is lower than that of island cotton, which shows the characteristics of high lint content and significant single boll.