A preparative method for carboxylic acids is disclosed in the present invention. The method is characterized in that: compounds (II) are reacted in the presence of hydrogen peroxide and base to produce target products (I), as represented by the following reaction scheme: wherein R1 is aryl, pyridyl, pyrimidyl, pyridazinyl, pyrazinyl, benzothienyl, benzofuranyl, quinolinyl, isoquinolinyl, thiadiazolyl, C1-6 alkyl, C3-6 cycloalkyl, C2-6 alkenyl, C2-6 alkynyl and hydrogen; R2 is alkoxycarbonyl, alkylaminocarbonyl, aminocarbonyl, alkylthiolcarbonyl, cyano, sulfonyl, sulfinyl, carbonyl, aldehyde, carboxyl, nitro, alkyl and hydrogen; R3 is alkoxycarbonyl, alkyl amido carbonyl, aminocarbonyl, cyano, sulfonyl, sulfinyl, carbonyl, carboxyl and nitro. The present invention has the following main benefits: cheap and readily available starting materials, safe processes, high yield, good quality, which facilitates industrial production.