The regression analysis showed that the optimal economic benefits of the processed-potato in Zhangye city could be obtained under the uses of 0.100 t/hm2 of nitrogen, 0.076 t/hm2 of phosphorus and 0.198 t/hm2 of potassium, respectively, with the corresponsive theoretic yields of 42.89 t/hm2, 40.98 t/hm2 and 43.58 t/hm2, respectively. With the increase of nitrogen, phosphorus and potassium fertilizers, the yield of processed-potato increased but the marginal yield, product value and profit, fertilizer contribution rate decreased, which were accord with the law of diminishing returns.