The invention belongs to the field of fine chemicals and in particular relates to a method for preparing 9-fluorenone by liquid-phase oxidation. The method comprises the steps of dissolving fluorene in N, N-dimethyl formamide, and then adding a catalyst, stirring at 30-40 DEG C and under atmospheric pressure, carrying out air oxidation, reacting for 0.5-3 hours, distilling, filtering, washing with water and drying to obtain 9-fluorenone of which the yield is 98% and the purity is 99-99.5%, wherein the feed weight ratio of fluorene to N, N-dimethylformamide is 1 to (2-5), and the feed molar ratio of fluorene to potassium hydroxide is 1 to (0.01-0.05). The solvent used in the method disclosed by the invention can be recycled without further processing. The method disclosed by the invention has the advantages of mild reaction conditions, simple operation, relatively small amount of solvent, high yield, low cost, low environmental pollution and relatively short reaction time, the solvent needs not to be further processed and can be recycled after being used and the method is suitable for large-scale industrial production.