We set up the fertilizer effect trial of "single-factors, five level" on red soil hilly region of southern Hunan. In order to find the best application of potassium fertilizers on sweet potato, we measured yield, nutrient content of sweet potato, and soil available nutrients, by combining field trials with laboratory analysis. The results indicated that potassium fertilizers could improve the sweet potato economic output, and fertilizer cost obviously. Meanwhile, the application of potassium fertilizers also improved the total amount absorption of Nitrogen， Phosphorus， and Potassium of potato tubers, cane leaves, and improved soil available potassium content significantly. Under the middle and low application amount of potassium, the soil available nitrogen and available phosphorus content was increased. By comprehensive consideration, in the four application of potassium fertilizers, the best application on sweet potato was 225 kg/hm2, the yield rate were 52.9 percent, the income per ha reached 47.7 percent.