Abstract: 【 Objective 】 This article revealed the intrinsic factor of fiber strength during cotton fiber development and an effective method to improve fiber strength by studying the mechanism differences between fiber orientation parameters and strength of cotton varieties in Northern Xinjiang. 【 Method 】3 upland cotton varieties were taken as the research object to collect boll samples and separate the fiber and seed by hand every 5 days from 30d post anthesis to boll opening stage. After that, the quality and orientation parameters (α、φ、ψ) of the fiber were measured. 【 Result 】With the development of fiber development, the orientation dispersion (α) of three upland cotton varieties increased first and then increased. The trend of gradual optimization was found in the early stage and the widening tendency was detected in the later period, which was detrimental to the formation of high strength fiber. The final α-angle of Xinluzao16 and Xinluzao10 was alike, and the final α-angle of Xinluzao 13 was the lowest. Spiral angle (φ) of three upland cotton varieties decreased with fiber development. The φ-angle was the highest during early fiber development and minished (optimize) step by step with fiber development. But it tended to be widened during late fiber development. The final φ-angle of Xinluzao 10 was the highest and the final φ-angle of Xinluzao16 was the lowest. There were obvious differences in φ-angle of different cotton varieties and the difference was maintained during fiber thickening development. The large differences of orientation distribution angle (ψ) existed in dynamic changes of three cotton varieties. The ψ-angle of Xinluzao13 had similar variation tendency with φ-angle. The ψ-angle of Xinluzao 16 and Xinluzao 10 increased first and then decreased with fiber development. The variation tendency of minishing (optimize) was terminated and started widening during late fiber development. The final ψ-angle was the same as φ-angle. The final ψ-angle of Xinluzao10 was the highest and the final ψ-angle of Xinluzao16 was the lowest. 【 Conclusion 】Spiral angle (φ) and orientational distribution angle(ψ) were the major factors to effect fiber strength during cotton fiber development. Forming a smaller structure of φ-angle and ψ-angle during early fiber development was one of the effective ways to improve fiber strength by cultivation measures such as chemical regulation in farm production.