The invention discloses a green preparation technology of phthalic dicarboxaldehyde, which takes 1, 3 -phthalan as raw material, hydrogen peroxide as an oxidant and immobilized rare earth metal and polyoxometallate (heteropoly acid) as catalysts, and water as solvent, and under the invigorating effect of ultrasonic field, the phthalic dicarboxaldehyde product is obtained through the processing steps of catalytic oxidation, ligarine recrystallization, suction filtration and the like. Compared with the prior art, the green preparation technology provided by the invention has the advantages as followings: the process reaction conditions are mild, the preparation process is simple, raw materials are easy to access, the reaction time is short, the product purity is high, zero contamination, small energy consumption and green economy are achieved, and the green preparation technology facilitates the popularization and the use of phthalic dicarboxaldehyde in the industries such as chemical engineering, medical and the like.