The invention relates to a method for preparing an imine from primary amine by photocatalytic oxidation at room temperature under the action of a ligand modified supported Au catalyst. The method comprises the following steps: adding the substrate primary amine, a solvent and the ligand modified supported Au catalyst to a reaction tube, introducing oxygen to replace internal atmosphere, and performing a reaction under visible light irradiation for 1-20 h; after the reaction, centrifugally separating the catalyst, performing reduced-pressure distillation to remove the solvent, and purifying anobtained sample through column separation. The conversion rate of the primary amine is 92%-100%, and the yield of the imine is 95%-99%. Oxygen is taken as an oxidizing agent in the catalysis process,and the method has the characteristics that the conversion rate and the selectivity are high, the operation is simple, the catalyst is recyclable and long in service life, environment-friendliness isrealized, and the like.