The invention relates to a preparation method of a first-class benzoxazine compound .The structural formula of the prepared benzoxazine compound is shown in the drawing, wherein R1 is a hydrogen atom, an alkyl group, an aryl group, halogen, an alkoxy group, an aryloxy group, a trifluoromethyl group and the like .The preparation method includes the steps that ortho-aminophenol and dibenzoyl serve as the reaction raw materials, 1,2-dichloroethane serves as the solvent, under the catalysis of acid, a molecular sieve is added, reaction is conducted at the temperature of 60 DEG C for 10 hours, after reaction ends, extraction and condensation are conducted, column chromatography isolation is conducted to obtain a benzoxazine compound intermediate, under the catalysis of alkali, 1,2-dichloroethane serves as the solvent, reaction is conducted at the temperature of 80 DEG C for 8 hours, condensation is conducted, and column chromatography purification is conducted to obtain the product compound .The yield is 60-80%, and alkali is sodium hydride, calcium hydride, potassium hydride, tertiary butanol alkali and the like.