The invention discloses a preparation method of an octenyl succinic acid water-soluble starch ester. The method comprises the following steps: grinding dry ore materials, screening and evenly mixing with octenyl succinic anhydride; then dripping a water-soluble starch solution into a mixture of the ore materials and the octenyl succinic anhydride and reacting under an alkaline condition; and finally, regulating a pH value to reach acidity, centrifuging after oscillating, taking out a supernatant and drying, thereby obtaining the octenyl succinic acid ester of the water-soluble starch. The method has the advantages that the octenyl succinic acid water-soluble starch ester with high substitution efficiency and low octenyl succinic acid residue quantity can be prepared by directly utilizing the water-soluble starch.