Factors influencing the degree of substitution (DS) of phosphate monoester of sweet potato starch with esterification of sodium phosphate monobasic (Na₂HPO₄) and sodium phosphate dibasic (NaH₂PO₄) were systematically investigated. The results indicated that the degree of substitution of phosphate monoester increased with the level of esterifying agent, reaction temperature, reaction time, and the amount of catalyzer. However, when pH value increased, DS first increased, then decreased. The combination of Na₂HPO₄ and NaH₂PO₄ and vacuum had no marked influence on DS. Based on above results, the optimum parameters for the preparation of phosphate monoester of sweet potato starch was selected as follows: the reaction temperature is 130～140℃, reaction time is 2～3 h, pH value is 5.5～6.0, the ratio of Na₂HPO₄ to NaH₂PO₄ is 3, and the concentration of catalyzer is 4%～6% of starch weight.