The invention discloses a method for preparing carboxylic ester by electrochemical dechlorination of a trichloromethylpyridine derivative. The method is characterized by including the following stepsthat (1) a polychloropyridine derivative is dissolved in an alcohol or phenol solution containing an electrolyte to obtain an electrolytic reaction solution; (2) with the electrolytic reaction solution as a catholyte, an electrolytic reduction dechlorination reaction is carried out at the cathode to obtain a polychloropyridine carboxylic acid ester derivative, wherein the polychloromethylpyridinederivative is shown as the formula (I), the product polychloropyridine carboxylic acid ester derivative is shown as the formula (II), in the formula (I), m is 0 or 1 or 2 or 3 or 4, and in the formula(II), m is the same as m in the formula (I) and R is C1-C8 alkyl or phenyl or functional-group-containing phenyl. The method has the main advantages that the reaction can be completed in one step atnormal temperature and normal pressure; no concentrated sulfuric acid is used in the reaction process, and thus the generation of a large amount of high-COD acidic waste liquid difficult to dispose isavoided; no high-toxicity thionyl chloride is used, and the generation of sulfur dioxide gas is avoided.