The invention discloses a method for preparing acetate starch with mechanical activation solid phase reaction, comprising the following steps: a, mixing acetic anhydride equivalent to 40-80% of starch mass with starch, then adding solid sodium hydroxide equivalent to 1.0-4.0% of starch mass, stirring uniformly and then sealing for placing for 12-36h; b, loading a mixture obtained in the step a in a self-made ball-stirring mill, adding a ball milling medium with a ball material volume ratio of (6-12): 1, wherein the diameter of the ball milling medium is 4-8mm, stirring reaction lasts 40-80 min at the temperature of 40-80 DEG C, and the rotating speed of strirrer is 200-500 rpm; and c, separating the product with the ball milling medium, washing with ethanol with the volume fraction of 60-90% till the pH value is 6.0-7.0, vacuumizing and filtering, drying at the temperature of 30-60 DEG C, and then crashing till 100-120 meshed. According to the method, the acetate starch with high substitution value can be prepared, and the technology is simple and environmentally-friendly.