The invention provides a preparation method of a phenoxycarboxylic acid herbicide, wherein the preparation method includes the steps: S1, mixing anhydrous phenol, alkaline substance and chlorocarboxylic ester, and carrying out condensation reaction by a one-pot method in an anhydrous system to obtain phenoxycarboxylic ester, wherein the chlorocarboxylic ester has the general formula of ClR1COOR, R1 is C1-3 alkylene or alkylidene, and R is C1-10 alkyl or C3-10 of cycloalkyl; S2, carrying out selective chlorination of the phenoxycarboxylic ester with a chlorinating agent in the presence of a first catalyst and a second catalyst to obtain chlorobenzoxycarboxylic ester, wherein the first catalyst is selected from Lewis acid, and the second catalyst is C5-22 thioether compounds, thiazole compounds, isothiazole compounds or thiophene compounds; and S3, carrying out acidolysis reaction of chlorobenzoxycarboxylic ester to obtain the phenoxycarboxylic acid herbicide represented by the formula I, wherein R3 is H, Cl or CH3. The preparation method can improve the product quality and the operation environment of the production site, and has low quantity of three wastes.