The invention discloses a preparation method of alkali metal methoxybenzoate shown in a formula B. The preparation method comprises the following steps: 1, under the protection of inert gas, performing nucleophilic substitution reaction on a compound A and sodium methylate in methanol, wherein the reaction temperature is 80-150 DEG C and the reaction pressure is 0.18-1.4 MPa; 2, directly mixing a solution obtained in the step 1 with alkali metal hydroxide and water, removing methanol, and performing hydrolysis reaction to obtain a compound B, wherein the reaction temperature is 90-190 DEG C, and alkali metal hydroxide is sodium hydroxide and / or potassium hydroxide; performing acidification reaction on the prepared compound B and acid to prepare methoxybenzoic acid shown in a formula C. According to the method, the raw materials are easily available, a low-price safe reagent is used, and a one-pot process is adopted; the method is low in alkali consumption, little in environmental pollution, easy to perform, high in yield and suitable for industrialized production.