Objective: This study researched the effects of the optimal proportion of Nitrogen, Phosphorus and Potassium to cotton yield under the same irrigation condition in arid area. Method: The experiment adopted orthogonal saturated designs, design of different N, P fertilization level and their combination treatment, and the total experimental plots were 27. Result: According to the experiment results, the polynomial regression was established, that is, the cotton (ginned cotton) yield (Y) and nitrogen (N) and phosphorus (P) quantity of regression equation: Y=206.58+3.21 XN +9.83 XP -0.003 5 XN2+0.029 XP2-0.011 XNXP . The optimum formulation of fertilizer was: nitrogen fertilizer was 260.74 kg/hm 2; phosphorus fertilizer was 120.07 kg/hm 2. At this time cotton (ginned cotton) can get the highest yield: Y=1 214.98 kg/hm 2. Conclusion: Different levels of nitrogen and phosphorous fertilizers decide different cotton yields. Interaction between Nitrogen and Phosphorus is very significant.
