Based on method of "two step selection" from 86 cotton (Gossypium hirsutum L.) cultivars from 2001 to 2003, an experiment was conducted to study potassium (K) efficiency of varied cotton genotypes through solution and soil pot culture. The result showed that K efficiency coefficients (KEC:K efficiency coefficients is the rate of low K level plant weight to high K level plant weight at seeding stage, or K efficiency coefficients is the rate of low K level cotton fiber yield to high K level cotton fiber yield during whole stage) existed prominent difference between various genotypes. The range of KEC was from 0.08 to 0.28 at seeding stage. According to agricultural character, KEC and different cotton pedigrees, 6 representative cultivars were selected for further experiment, and after raising low K level selection on these 6 cultivars, 4 typical cultivars were selected for whole growth stage experiment. Together with other indices, such as agricultural characters, fiber yields, yield potential and so on, cotton fiber yield and KEC (range from 0.24 to 0.39) were used to identify the real K high efficiency genotype. Finally 103 was regarded as the K high efficiency and high yield potential genotype, 122 as the K low efficiency and low yield potential genotype, 163 and 165 as the K high efficiency and low yield potential genotypes. 103 could get much more yield with low K content at the same K level, so it had stronger ability to make full use of plant potassium than other genotypes. Besides, 103 could absorb much soil rapidly available potassium and activate soil slowly available potassium, so 103 had higher absorption and utilization abilities to soil potassium, all of these maybe explain why 103 was the K high efficiency and high yield potential genotype.