Pot experiment was carried out in 2017～2018 to study the effects of topdressing ammonium nitrogen fertilizer and nitrate nitrogen fertilizer at initial flowering stage on growth and yield of cotton under six soils with different available potassium content (71.1，79.7，86.8，95.5，122.6，154.1 mg·kg-1).A split-plot design was used in the experiment with the main plots assigned to soil available potassium contents，and the subplots to topdressing nitrogen(N)fertilizer (ammonium sulfate and calcium nitrate)under the equal N conditions at initial flowering stage.The results showed that soil available potassium content significantly affected dry matter weight，the number of bolls and seed cotton yield of single cotton plant in two years，but had no significant effects on single boll weight.Dry matter weight，the number of bolls and seed cotton yield of single cotton plant increased in opening downward quadratic curve with the increase of soil available potassium content.The effects of topdressing ammonium sulfate and calcium nitrate at initial flowering stage on dry matter weight，the number of bolls，boll weight and seed cotton yield of single cotton plant were not significant.The seed cotton yield of single cotton plant for topdressing calcium nitrate at initial flowering stage in 2017 and 2018 was 5.6 % and 1.9 % higher than that of topdressing ammonium sulfate，respectively.There were no significant interactions between soil available potassium content and topdressing calcium nitrate and ammonium sulfate at initial flowering stage on dry matter weight，the number of boll，boll weight and seed cotton yield of single cotton plant in two years.The results provided a theoretical basis for rational application of N fertilizer of cotton under different soil potassium fertility conditions.