According to the results of 68 field experiments on N, P and K fertilizer application in the middle and lower reaches of Yangtze River from 2009 to 2014, N, P and K fertilizer application index system was established for sweet potato, which was based on nutrient abundance and deficiency index method and fertilizer effect function. The results show that application of N, P and K fertilizers have significant effects on increasing yield of sweet potato (P<0.01). For potato-growing soil, the “High” and “Low” indexes are 170 mg/kgand 100 mg/kg for available (hydrolytic) nitrogen, 110 mg/kg and 30 mg/kg for available potassium, respectively, while the critical index is 17 mg/kg for available phosphorus. The optimal economic recommended rates of nitrogen, phosphorus and potassium are N125 kg/hm², P₂O₅160 kg/hm² and K₂O182 kg/hm², respectively, the optimal economic recommended application rates of N, P and K for medium and low yield fields are N155 kg/hm², P₂O₅120 kg/hm² and K₂O220kg/hm², respectively.