Abstract: In order to better explore the response mechanism of different cotton varieties to phosphorus under low phosphorus stress, and 21 cotton varieties of different genotypes were treated with high phosphorus(KH₂PO₄ 1×10^-3 mol/L)and low phosphorus(KH₂PO₄ 1×10^-5 mol/L)by hydroponic method, the effects of different phosphorus contents on root morphology of cotton seedlings were studied through statistical methods like principal component analysis. The results showed that the root volume and root surface area of cotton increased significantly after low phosphorus treatment, and the root growth of cotton decreased with the decrease of phosphorus content. Through screening, 1 high-phosphorus cotton variety, 2 medium-phosphorus cotton varieties and 18 low-phosphorus cotton varieties were obtained. It was also found that there were significant differences in total root length, root area, root surface area and root volume among different phosphorus treatments and varieties, which could be used to evaluate the phosphorus tolerance of cotton varieties.