The invention discloses a method for preparing 1,4-dihydrooxazine. An acyl diazo derivative and an acridine compound are taken as reaction substrates, AgOTf and Cu(OAc)2 are taken as reaction catalysts, NaHCO3 is used as an alkali, and the 1,4-dihydrooxazine is obtained through a ring addition reaction in an organic solvent. The method provided by the invention has the following characteristics that the reaction is more economical, functionalization in the later period is easier, reaction conditions are mild, the reaction can be carried out in the air, the use amount of the catalysts is small,post-treatment is simple and convenient, purification and large-scale industrial application of the product are facilitated, and one-step functionalization of commercial medicines is easier to carryout. Meanwhile, the raw materials such as the reactants, the catalysts and the like are cheap and easily available, the reaction composition is reasonable, a ligand is not needed, the atom economy ishigh, the reaction steps are few, and a relatively high yield can be obtained only by the one-step reaction, so that the method accords with requirements and directions of contemporary green chemistryand pharmaceutical chemistry and is suitable for screening high-activity 1,4-dihydrooxazine medicines, a gram-grade reaction can be well realized, and the method is suitable for large-scale industrial production.