Abstract: Mechanical harvesting technology of cotton has been increasing in modern agriculture in China, particularly for cotton serving as an important strategic reserve material. In mechanized harvesting of cotton, mechanical properties of machine-harvested cottons have become the key factors to cotton harvesting, even to the design of harvesting machinery. The machine-harvested cotton can fall off early in a bad weather after maturity, due mainly to the physical parameters of boll shell, resulting in the loss of pre-harvest cotton. Sometimes the cotton cannot be completely harvested, but the cotton can be knocked off when mechanized harvesting, resulting in the losses during harvest. The physical parameters of boll shell can be closely related to the mechanical effect of working parts, when the cotton is mechanically harvested, with emphasis on the separation force between the boll shell and cotton. Therefore, it is necessary to reveal the effect of physical parameters of boll shell on mechanical properties of machine-harvested cotton, particularly when designing cotton harvesting machinery. In this study, 3 kinds of representative machine-harvested cottons were selected as the research objects, including Xinluzao 45, Xinluzao 66, and Xinluzao 83, collected from the Shihezi area of Xinjiang, China. A laboratory tensile separation test was used to assess the mass fraction of boll shell, carpel angle, and lock angle for the 3 kinds of machine-harvested cottons under the same growth conditions. The experimental results showed that the separation force between cotton and boll shell were 0.155-0.980 N, 0.275-0.967 N, 0.258-0.667 N, for the Xinluzao 45, 66, and 83 machine-harvested cottons, respectively. Three stage can be divided for the machine-harvested cottons, including the elastic-like stage, yield-like stage, and separation stage, in the laboratory tensile separation tests. In the yield-like stage, the tensile separation curve of cotton showed the zigzag fluctuation with the multiple peaks. It inferred that there were concurrently elastic and plastic deformation of cotton fiber with the dominance of plastic deformation. The internal interlaced and intertwined fiber of cotton tissue can produce local dislocation, when the external load of cotton flower reached a critical value. The mass fraction of boll shell, carpel angle, and lock angle in the machine-harvested cottons have a significant effect on the separation force of cotton from boll shell (P<0.01). A power function relationship can be found between the separation force of cotton boll shells and the carpel angle, as well as the mass fraction of boll shell (P<0.001). The separation force of cotton and boll shell gradually decreased, with the increase in carpel angle and mass fraction of boll shell. There was a significantly negative correlation between the lock angle and the separation force, indicating that the separation force of cotton and boll shell gradually decreased with the increase of lock angle. A recommendation was made that a relatively small weight ratio of shell and a uniform locking angle can be selected, when breeding cotton crops for machine harvesting. The cotton can be timely harvested after maturity, in order to avoid the losses before harvest, when choosing the harvesting time. The finding can provide a sound theoretical and practical guidance for the cultivation of new harvesting cotton, as well as for the design and optimization for the harvesting machinery of cotton.