In order to study the effect of interaction of nitrogen (N) and potassium (K) fertilizer on cotton yield and nitrogen use efficiency(NUE)and assessment of suitable fertilization level in the Yangtze River Basin，field trials were conducted in 2017 at the Vegetable Research Institute of Wuhan Academy of Agricultural Sciences，Huangpi District，Wuhan，and in 2018 at Datong Lake Farm，Wuhan，Hubei.Samples of aboveground cotton plants were taken in early July，early August and early October，respectively.The stems，leaves，buds and bolls were collected from 3 representative plants selected from each plot.N content in different organs and yield were determined to study the effects of combined application of N and K fertilizers on NUE and yield.The results showed that the average increase rate of cotton yield with N fertilizer was 58%～89% without K supply，and 65%～141% with K supply.Correspondingly，the combined application of N and K significantly increased the NUE.Compared with no K supply，the NUE of N fertilizer with K supply increased by 14.6%.In general，the combination of N and K fertilizer can significantly promote the growth and yield of cotton，improve NUE，and reduce the pollution caused by overuse of N fertilizer.The optimum N and K fertilizer rates for high cotton yield and low N loss in the Yangtze River Basin were 190～375 kg·hm-2 with an average of 283 kg·hm-2，and 56.5～185.0 kg·hm-2 with an average of 120 kg·hm-2，respectively.