Abstract: 【Objectives】 The aim for this study was to define effect of potassium application rate on crop yields and potassium use efficiencies in a wheat-cotton double cropping system, and to determine the highest yield and economic optimum potassium fertilizer rate for the two crops. The results could provide evidence for suitable potassium applied amount in the wheat and cotton cropping system of North China Plain. 【Methods】 The field experiments with five treatments of total K2O application rates(0, 105, 210, 315 and 420 kg/hm2) were conducted in Shangqiu city and Neihuang county, Henan province from 2008 to 2009. The single-season K2O application rates for wheat and/or cotton were 50% of the total, and the nitrogen and phosphate fertilizers for each treatment were the same. Through analyzing the difference of crop yield, output value and increased benefit of wheat and cotton, and potassium use efficiency(KUE) in the five potassium treatments, the maximum yield and economic optimum potassium application rate of wheat-cotton were calculated using potassium mathematical modeling.【Results】1) Compared to K2O 0 kg/hm2, the wheat yield for the other four potassium treatments are increased by 6.6%-9.8% and 7.2%-8.9% in the two experimental sites, respectively, and the highest yield is obtained at the potassium rate of 105 kg/hm2, whereas there are no differences among the four potassium treatments. In addition, wheat grains per spike and 1000-grain weights are increased significantly in the four potassium treatments(P＜0.05), whereas no significant differences are observed in the effective spike numbers. Moreover, the optimum wheat yields are obtained at the potassium application rates of 75.7 kg/hm2 and 63.9 kg/hm2, respectively, in the two experimental sites, whereas the maximum wheat yields appear at the potassium application rates of 143.2 kg/hm2 and 111.6 kg/hm2, respectively. 2) Compared to K2O 0 kg/hm2, the cotton yields for the other four potassium treatments are increased by 42.3%-52.5% and 10.9%-15.6% in the two experimental sites, respectively, and the highest yields are obtained at the potassium rate of 105 kg/hm2, whereas there are no differences among the four potassium treatments. In addition, The boll numbers per plant and boll weights are increased significantly at the four potassium treatments(P＜0.05), whereas no significant differences are observed in lint percentage. Moreover, the optimum wheat yields are obtained at the potassium application rates of 106.9 kg/hm2 and 111.3 kg/hm2, respectively, in the two experimental sites, whereas the maximum wheat yields appear at the potassium application rates of 113.2 kg/hm2 and 138.0 kg/hm2, respectively. 3) The output values and increased benefits of wheat and cotton are increased significantly in the four potassium treatments, and the highest values are obtained at the potassium rates of 210 kg/hm2. However, there are no significant differences among the four potassium treatments, except the treatment of K2O 0 kg /hm2. At the experimental site of Shangqiu city, the potassium apparent recovery efficiency(KARE) of wheat is decreased with the increase of potassium application rate. At the experimental site of Neihuang county, the highest KARE of wheat is obtained at the treatment of 105 kg/hm2, and then the KARE of wheat is declined with the potassium application increasing. With the potassium application increasing, the accumulation amounts of nitrogen, phosphorus and potassium for both wheat and cotton are increased within a certain amount of potassium, but over this range, their accumulation amounts are steady-going. The total KARE in the wheat-cotton double cropping system is decreased with the increase of potassium application rate, as well as the potassium partial factor productivities and agronomic efficiencies of wheat and cotton. 【Conclusion】 In this study, the yields of wheat and cotton are increased with the increase of potassium application rate at a suitable range, and then decline or have a unchanged trend. The crop yields for both wheat and cotton are improved, resulting from grains per spike and 1000-grain weight increasing significantly in parallel with boll number per plant and boll weight. However, the total KARE in the wheat-cotton double cropping system is decreased with the increase of potassium application rate.