The invention discloses thermochemical modified starch, a preparation method and a method for preparing an adhesive by using the thermochemical modified starch and relates to modified starch and adhesives prepared by using the starch. The method is used for solving the technical problems of the existing starch adhesives that the water resistance is not ideal, the transportation cost is high, and the starch modification process is complicated. The thermochemical modified starch is prepared through uniformly spraying an acid solution into the starch, carrying out uniform mixing, then, carrying out heating treatment, carrying out baking, carrying out cooling, and then, carrying out crushing. The method for preparing the adhesive comprises the following steps: adding polyvinyl alcohol into a reactor pot with stirring, heating and cooling devices, adding water, adopting a staged-heating method to dissolve the polyvinyl alcohol with stirring, and then, cooling to room temperature; adding the thermochemical modified starch, carrying out stirring at room temperature, and then, adding polymethylene polyphenyl polyisocyanate, thereby obtaining the adhesive. According to the method, by using the technological characteristic that the starch-based adhesive is prepared by only adding the water to the thermochemical modified starch before use, the problem that the traditional solution type starch main agents are low in transportation efficiency and are prone to layering and mildewing is solved.