Abstract: To investigate the content of free and bound phenol in different varieties of sweet potato, "Liaoshu 20" purple sweet potato lyophilized powder whose free phenol had been extracted adequately was used as the raw material to study the extraction of the bound phenol by acid hydrolysis. Based on the single factor test, H₂SO₄ with volume fraction of 10% was utilized to hydrolyze the material and the response surface analysis method were used to optimize the relevant parameters. Then, free and bound phenol in twelve variety of sweet potato were extracted and determined. The results showed that the optimal parameters for the extraction of bound phenol by acid hydrolysis were:Extracting time 22 h, temperature 62℃, ratio of liquid to solid 35:1 mL/g, the actual extraction amount of (0.212±0.004) g/100 g, reaching the predicted value (0.220 g/100 g) of 96.36%. The content of free phenol was more than that of bound state (average content is 0.61 and 0.15 g/100 g respectively) and both of them in these twelve varieties of sweet potato was significantly different with wide range of (0.14~1.63) and (0.05~0.28) g/100 g. Compared with other color fleshes of sweet potato, purple flesh varieties have more free and bound phenol. These data suggested that this bound phenol extraction technology was practical and purple flesh varieties were better material for extraction of free and bound phenol. This study laid the foundation for evaluating the total amount of polyphenols in different varieties of sweet potato and further researching the bioactivity of bound phenol from suitable sweet potato varieties.