Abstract: Taking corn starch as a raw material,sodium tripolyphosphate as an esterifying reagent,urea as an catalyst,phosphate starch was synthesized by a half-dryprocess. The effects of the amounts of sterifying reagent and catalyst,pH,reaction temperature and time on combined degree of substitution and viscosity were investigated. The results indicated that the best reaction conditions were:amounts of tripolyphosphate and urea was 6.0% and 5.0%,pH8.0,reaction temperature 135 ℃,reaction time 2.0 hours. Under these conditions,the gelatinization temperature was 55.4 ℃,the peak viscosity was 2084 BU,the degree of substitution was 0.0201%. The Pasting property was higher than corn starch. It could greatly improve its application in food and paper industry.