Abstract: Relationship between applying K fertilizer and the yield of sweet potato was studied with variety of Xicheng Potato 007.The results showed that: K fertilizer improved the assimilative capacity of the sweet potato,and increased the dry matter accumulation while the plants partly inhibited in excessive growth of the stems and leaves.K fertilizer enhanced the plant capability in transporting assimilate to underground tuber,by which reducing the value of T/R and increasing economic production.Root dry rate and stem diameter were related with the species characteristics of sweet potato instead of K fertilizer amount.Reducing K fertilizer amount increased the portion in large-and medium-sized sweet potato,the length of the longest stem and the number of base branches.Comparing to control,the dry matter content was decreased by applying K fertilizer.The chlorophyll content of plants was higher in those treated with N and P fertilizer than with K fertilizer and without fertilization.For a low K level soil,the optimal amount of K fertilizer was 253.05-262.46 kg·hm-2 for producing high yield of dried sweet potato and starch,and was 316.05-321.59 kg·hm-2 for producing high yield of fresh potato tuber yield.There was no difference observed between low K and high K levels on dry matter production,although K fertilizer improved fresh yield effectively.