The invention discloses 3-cyclopropylmethoxy-4-halogen benzoic acid or a derivative (I) and an application thereof. The 3-cyclopropylmethoxy-4-halogen benzoic acid or the derivative (I) can be used for preparing roflumilast and has the advantages that raw materials are easy to obtain, the reaction condition is mild, the operation is simple and convenient, complicated postprocessing processes such as column chromatography, high-temperature rectification and the like are avoided. 3-cyclopropylmethoxy-4-difluoromethoxybenzoic acid (IV) prepared by a compound (I) is subjected to a condensation reaction to obtain the roflumilast, intermediates in each step and goal products are high in purity, the quality is stable and controllable, the preparation cost of the roflumilast is greatly reduced, and the 3-cyclopropylmethoxy-4-halogen benzoic acid or the derivative (I) is suitable for industrialization large-scale preparation. A general molecular formula of the compound (I) is shown as follows.