The invention discloses a method for preparing 2,3-dihydroquinazolin-4(1H)-one and its derivatives, belonging to the technical field of organic chemical synthesis. In the preparation reaction, the molar ratio of isatoic anhydride, aromatic amine and aromatic aldehyde is 1:1:1, the molar amount of high-acidity ionic liquid catalyst is 8-12% of the isatoic anhydride used, and the reflux reaction time is 4-30min , the volume of the reaction solvent water and ethanol mixture is 3 to 6 times the molar weight of isatoic anhydride, and the reaction pressure is one atmospheric pressure. After the reaction is completed, it is cooled to room temperature and filtered with suction, and the obtained filter residue is vacuum-dried to obtain pure 2,3- Dihydroquinazolin-4(1H)-one and its derivatives. Compared with the preparation methods using other catalysts, the present invention has the characteristics of less catalyst toxicity, recyclable use, better biodegradability, green and economical whole preparation process, simple and convenient operation, and convenient industrialized large-scale production.