Molecular geometries,electronic structures,heats of formation of the reactants, products and activated complexes for a whole N-H Mannich reaction involving benzoxazolon as pseudo acid component were calculated by semiempirical AM1 quantum mechanical method. According to the calculated activation energies,the mechanism of reaction in acidic media is suggested that formaldehyde reacts with benzoxazolon firstly，then the intermediate reacts with dimethylamine to form Mannich base, and the reason is discussed herein.