Abstract: In order to clarify the physiological basis of increasing soluble sugar content in storage roots of edible sweet potato by potassium fertilization, the typical cultivar of edible sweet potato (Beijing 553) was grown in a replicated experiment at the Agricultural Experiment Station of Shandong Agricultural University in 2 summer crop seasons of 2009 and 2010. The treatments included the control and treatments with different dosages of potassium. The changes of starch content, soluble sugar content and related enzyme activities of storage roots, sucrose content and related enzyme activities of functional leaves were determined by using periodic sampling during the enlargement of sweet potato storage roots. The results show that compared with the control, the application of potassium can increase yield, the content of soluble sugar and its components of storage roots, and the largest increase appears in the treatment with 24 g K2O. In other words, the dosage of this treatment is the optimum one. The sucrose phosphate synthase (SPS) activity and sucrose content of functional leaves are increased significantly, with the average increase of 10.31% and 34.13%, and the sucrose synthase (SS) activity and insoluble acid invertase activity of storage roots are increased observably with the average increase of 16.47% and 3.66% during whole growth period in the treatment with the optimum dosage. These results indicate the transports of photosynthate from functional leaves to storage roots are improved. As a result, the accumulations of soluble sugar and starch in storage roots are achieved. Meanwhile the amylase activities in the treatment with the optimum dosage are increased significantly with the average increase of 27.90% and 14.26%, respectively for - and -amylase, which enhances the conversion from starch to soluble sugar. Besides, the treatment with the optimum dosage can increase activity of soluble acid invertase during the pre and post growth periods of sweet potato, and can increase sucrose: sucrose 1-fructosyltransferase (SST) activity at the later growth stage of sweet potato as well. The improvement of soluble acid invertase activity and SST activity is benefit to the accumulation of glucose and fructose in storage roots. At the harvest, the contents of soluble sugar and starch are increased by 13.52% and 3.02% respectively. To sum up, the physiological reasons of increasing soluble sugar content in storage roots caused by potassium application are that it can increase supply of sucrose, promote absorption of sucrose and promote hydrolysis of starch.