The invention discloses a method for preparing 2-ethyl hexanoic acid by catalytically oxidizing 2-ethylhexanal by molybdovanadophosphoric acid, and relates to a method for preparing a chemical preparation. The method comprises the following steps: weighing molybdovanadophosphoric acid and dissolving the molybdovanadophosphoric acid in distilled water at first and then in hydrochloric acid to prepare a catalyst solution; adding 2-ethylhexanal and the prepared catalyst in an ordinary-pressure reaction device, slowly heating and stirring the mixture, regulating oxygen flux, and starting to introduce oxygen when the temperature rises to the reaction temperature; after the reaction is ended, separating out the aqueous phase solution on the lower layer to obtain organic phase matter on the upper layer; and depressurizing and distilling the organic phase matter to obtain colorless transparent and pungent oily liquid, 2-ethyl hexanoic acid. By changing the ratio of molybdenum to vanadium of the catalyst and optimizing the reaction condition, the conversion rate of the 2-ethylhexanal reaches above 99%, and the selection and yield of the 2-ethyl hexanoic acid reach above 98% respectively.