The invention discloses a method for preparing starch octenyl succinate. The method comprises the following steps of: carrying out injection cooking on pre-emulsion comprising starch, alkali, water, octenyl succinic anhydride and a low boiling solvent to obtain cooked slurry, and carrying out enzymatic hydrolysis to obtain the starch octenyl succinate. According to the method, reaction is carriedout under the condition of injection cooking and the action of high-temperature water vapor, so that intermolecular hydrogen bonds and crystal structures are rapidly destroyed; and due to the remarkable gelatinization process, starch molecular chains are sufficiently swollen, and the amorphous region of the starch is significantly increased. At the same time, the high temperature water vapor can also enable the low boiling solvent in the system to be rapidly removed; the octenyl succinic anhydride is rapidly and irregularly dispersed and separated out in the system, and is in full contact withstarch granules for reaction in the high-speed liquid flow state of an ejector, so that the octenyl succinic anhydride undergoes esterification reaction at various positions of the starch molecule, and a plurality of sites of the starch are significantly substituted; and the obtained starch octenyl succinate has relatively high substitution degree and emulsifying property.