The invention discloses an octenyl succinic starch ester preparation method. The method includes: (1) subjecting alkaline dispersion liquid of starch and oil solution of octenyl succinic anhydride tojet cooking to obtain cooking liquor, or subjecting alkaline mixed liquid containing starch, octenyl succinic anhydride, water and grease to jet cooking to obtain cooling liquor; (2) standing the cooking liquor obtained in the step (1) to realize layering, taking lower emulsion, performing enzymolysis to obtain enzymolysis liquid, and subjecting the enzymolysis liquid to aftertreatment to obtain octenyl succinic starch ester. The preparation method is simple and efficient, and high replacement rate of the obtained octenyl succinic starch ester is achieved.