The invention discloses a method for preparing a chloromethyl pyridine derivative by electrochemical selective dechloridation. The method comprises the following steps: taking an acidic solution as a reaction medium, and adding a trichloromethyl pyridine derivative as shown in formula (I) into the acidic solution to obtain an electrolytic reaction solution; carrying out electrolytic reaction in an electrolytic cell which takes a carbon material as a cathode and takes a chemical inert conductive material or a titanium metal material coated with a noble metal oxide as an anode at a temperature being 20-40 DEG C, wherein the current density is 0.5-10A/dm<2>, and the pH is equal to 1-6; and after the electrolytic reaction is ended, separating and purifying to obtain the chloromethyl pyridine derivative as shown in formula (II). The method realizes carrying out hydrogenation reduction on the trichloromethyl pyridine derivative with relatively high chemical selection (greater than or equal to 75%) and a relatively high yield (greater than or equal to 70%) to obtain the chloromethyl pyridine derivative, so that mercury and zinc powder with high toxicity are avoided, constant-current electrolysis is realized, and electrolytic potential is not needed to be controlled, and therefore, the method is beneficial for industrial production.