The invention discloses a method for preparing 3-cyano-4-isopropoxybenzoic acid. A first step, 3-formyl-4-methyl hydroxybenzoate is prepared with methyl parahydroxybenzoate; a second step, 3-cyano-4-methyl hydroxybenzoate is prepared with the 3-formyl-4-methyl hydroxybenzoate; a third step, 3-cyano-4-methyl isopropoxybenzoate is prepared with the 3-cyano-4-methyl hydroxybenzoate; a fourth step, the 3-cyano-4-isopropoxybenzoic acid is prepared with the 3-cyano-4-methyl isopropoxybenzoate. The preparation method of the 3-cyano-4-isopropoxybenzoic acid, which is provided by the invention, has the advantages that 1) the use of the cuprous cyanide of hypertoxic cyanide in a previous process is avoided, and the method is more suitable for industrialization; 2) cyano is prepared by formyl, and the method is high-efficiency and direct, and is suitable for quantity production; 3) all raw materials are cheap and easily obtained, and the cost advantage is obvious.