Abstract: 【Objectives】Potassium (K) is one of the important nutritional elements, and is necessary for plant growth and development. While proportion of bioavailabe K in soil K pool is relatively low and it is difficult for crop to effectively use, and shortage of K resources is a problem that the world needs to solve. K deficiency can affect cotton growth but sodium(Na) can share some physiological functions with K, so studying synergistic and substitution effects of Na and K is an effective way to improve the crop K efficiency. In this research, a solution experiment was conducted to study the substitution effects of Na and K on K use efficiency of two typical K-efficiency cotton genotypes, and to provide scientific and rational use of potash in the production process.【Methods】 To study the agronomic traits(root length, stem length and leaf numbers), dry weights, K and Na contents, K and Na accumulation and distribution in plant organs respectively and to explore the substitution effects of Na and K on K use efficiency of two typical K-efficiency cotton genotypes, a hydroponics experiment was conducted with different K and Na treatments in Huazhong Agricultural University. 【Results】 In deficient K, Na can increase the root lengths of both genotypes, and the increase amplitude in cultivar 103 is higher than in cultivar 122. The dry weights of each part of 103 and 122 are increased by the Na application. The K contents in both the roots and stems of 103 and 122 are also reduced, but the difference in the K accumulation between the two cultivars is not significant. Na application significantly improves the whole plant K use efficiency of 103, which is about 1.37 times of that under the lack of sodium. In adequate K condition, Na could also increase the root lengths of both genotypes, and the dry weights of each part of both genotypes are significantly increased. Meanwhile, the K content in roots and leaves is significantly reduced, the K accumulation amounts in all the parts except for leaves and stalks are increased in different degrees. When K is adequate, Na is beneficial to the increase of K use efficiency in 103 and 122, and the increase in 103 is 28%, which is higher than that in 122 (19%). Meanwhile, significant and positive interaction between K and Na is showed on the relative growth rates of roots and stems, the dry weights of all the parts, the K and Na contents and the accumulations in roots and leaves and the whole plant K use efficiency. 【Conclusions】 The root lengths under the addition of Na in deficient or adequate K are significantly enhanced by promoting root elongation to raise the absorption of K and biomass accumulation. In deficient K, Na could significantly improve K use efficiency of 103, while with the addition of Na in adequate K, the increase amplitude of K use efficiency of 103 is higher than that of 122. These results show that 103 has better K and Na replacement and synergistic effect.