Abstract: 【Objective】 This study aims to investigate response of the main growth and development traits of cotton seedlings to soil available potassium levels in order to provide a basis for formulating evaluation indicators of soil available potassium in cotton fields. 【Methods】 The variety 100 of the Institute of Cotton Research was used as the experimental material, plastic boxes with a length of 0.8 m, a width of 0.58 m, and a depth of 0.44 m were used, each box containing 240 kg of soil. Eight available potassium levels of soil were set by adding potassium fertilizer, including 68.1, 77. 4, 93.3, 104.2, 122.9, 130.9, 142.4, and 171.1 mg/kg (represented by K₁, K₂, K₃, K₄, K₅, K₆, K₇, and K₈, respectively). Sampling was conducted at 5 times during the cotton 1 leaf stage, 3 leaf stage, 5 leaf stage, 7 leaf stage, and bud stage to determine the agronomic traits, dry matter weight, total potassium content, and photosynthetic indicators of the cotton plant. 【Results】 During the first leaf stage, there was no significant difference in cotton plant height and stem diameter among different soil available potassium levels; During the 3-leaf stage, 5-leaf stage, 7-leaf stage, and budding stages, the plant height and stem diameter of cotton showed an increasing trend with the increase of soil available potassium levels; When the soil available potassium reached the levels of K₄ and K₅, the cotton plant height and stem diameter tended to stabilize. The leaf area and dry matter weight of treatments with K₅ and above were significantly higher than those of other treatments, but there was no significant difference between these treatments. At the bud stage, the E, Ci, Gs, SPAD values of functional leaves of cotton plants in each treatment showed no significant response to soil potassium levels. In terms of Pn, K₇ was significantly superior to K₂ and K₃. 【Conclusion】 When the soil available potassium level is higher than 123 mg/kg, it can promote the nutrient absorption and dry matter accumulation of cotton seedlings, promote the growth of cotton stems, increase leaf area, accelerate the growth and development process of cotton seedlings, and facilitate the transformation of cotton from nutritional growth to reproductive growth.