The invention provides a 2,4-dichlorophenoxyacetic acid preparation method including the following steps: A) providing phenoxyacetic acid; B) mixing hydrochloric acid, an oxidizing agent and the phenoxyacetic acid of the step A), carrying out a chlorination reaction in the presence of a chlorination catalyst, to obtain 2,4-dichlorophenoxyacetic acid, wherein the molar ratio of the phenoxyacetic acid to the hydrochloric acid to the oxidizing agent is 1 to (2-200) to (2-200), and the molar ratio of the hydrochloric acid to the oxidizing agent is 1 to (0.1-10). The 2,4-dichlorophenoxyacetic acid preparation method provided by the invention only needs one-step chlorination reaction, and then the 2,4-dichlorophenoxyacetic acid can be obtained; while the purity and yield of the 2,4-dichlorophenoxyacetic acid are ensured, the production process of the 2,4-dichlorophenoxyacetic acid is simplified. Moreover, the preparation method provided by the invention has no 'three wastes' emissions and achieves greenization of synthesis of the product.