The present invention relates to a purification method of hydroxypropylmethyl cellulose phthalate, more particularly to a method of preparing high-purity hydroxypropylmethyl cellulose phthalate through a simple crushing process which comprises: increasing fluidity of reaction product mixture by adding fluidization solvent as a post-treatment process; and spraying it into water through a spray nozzle. As a result, formation of fine granular hydroxypropylmethyl cellulose phthalate particles prevents inter-particle coagulation during the a post-treatment process, and phthalic anhydride reactant, free phthalic acid and remaining acetic acid solvent can be removed effectively.