The invention provides a method for preparing a quinolinone compound. The method comprises the following steps: by taking substituted oxamic acid and substituted ethylene as raw materials for a firsttime, in the presence of an oxidant and a catalytic amount of silver nitrate, stirring a mixed reaction liquid at a certain temperature of 50-100 DEG C for 36-48 hours in an organic solvent and water,extracting a product, drying, performing vacuum concentration, and performing column chromatography, thereby obtaining a 3,4-dihydroquinoline-2(1H)-ketone derivative. The method is simple and easy inraw material obtaining, gentle in reaction condition, simple to operate, high in yield and very high in application value.