The sweet potato starch was modified by isoamylase, and the natural polysaccharide was used instead of alum as additive to study the process of making sweet potato powder without alum. Moreover, several key factors including water quantity, heating temperature, polysaccharide addition amount and aging time were optimized. The effects of different conditions on the properties of sweet potato starch were examined by using gelatinization characteristic value, gel properties, cooking properties, shear force, breaking rate and tensile force of sweet potato starch as indexes. The results showed that isoamylase could increase amylose content and enhance starch gel properties, and guar gum could effectively replace alum and improve the quality of sweet potato powder. The order of the influence of each factor on the index was as follows. From high to low, they were water addition, heating temperature, aging time, and addition amount of guar bean gum. The optimal process combination was found to be adding 80% water and 0.3% guar bean gum, heating at 90 ℃, and aging for 10 h.