The invention discloses a mechanical activation reactor capable of enhancing polysaccharide high polymer modification. An outer barrel is in a horizontal structure and a grinding inner barrel is arranged in the outer barrel; a speed reducer I is connected with a stirring shaft by a shaft coupler; the stirring shaft penetrates through a through hole in an end cover of the outer barrel and is inserted into the grinding inner barrel; the stirring shaft is fixed at the two ends of the outer barrel by bearing seats at the two ends; a jacket is arranged outside the grinding inner barrel; a material inlet/outlet is formed in the outer barrel and is communicated with the grinding inner barrel; the grinding inner barrel is internally provided with grinding balls; the stirring shaft is provided with paddles and a material scraping screw belt; the paddles are mounted on the stirring shaft in a staggered manner; and the material scraping screw belt is mounted on the stirring shaft. The mechanical activation reactor is provided with the paddles and the material scraping screw belt so that the grinding balls in the grinding inner barrel can be driven by the paddles and the material scraping screw belt to move back and forth, and the disturbance of the grinding balls is enhanced, and furthermore, materials can react more uniformly and the surface updating is more rapid; and the reaction time is effectively shortened, the loss of the raw materials is reduced and the quality of a product is improved.