Abstract: The sago starch was mechanically activated by a stirring-type ball mill as pretreatment, and the octenyl succinic acid starch ester was prepared by the esterification agent octenyl succinic anhydride. By adopting the degree of substitution (DS) as evaluating parameters, activation time, reaction time, the amount of octenyl succinic anhydride, reaction tem-perature, and pH on the esterification of sago starch were investigated respectively. The results indicated that mechanical activation significantly improved the esterification of sago starch octenyl succinic anhydride. Under the same reaction conditions, the degree of substitution and reaction efficiency of activated starch increased significantly. Through the orthogonal test, the optimal conditions for the preparation of activated 1.0 h starch ester could be determined: The reaction time was 2.0 h, the amount of octenyl succinic anhydride was 3%, the pH value was 8.0, and the temperature was 35 ℃. Under such conditions, the degree of substitution of starch ester prepared by esterification was 0.02294, the reaction efficiency was 98.01%. The results of the FTIR, XRD and SEM indicated that the FTIR spectrum of octenyl succinic anhydride sago starch showed new absorption peaks at 1570 cm⁻¹ and 1712 cm⁻¹, the crystallinity of starch decreased, and the surface of starch granules was damaged, with large holes appeared in the middle of starch granules. Thus the esterification reaction of sago starch was further confirmed. The emulsifying property of octenyl succnic anhydride modified strach with substitution degree of 0.02294 was 26.43%, and the emulsifying stability was 24.10%, which were superior to those of the original sago starch ester.