The objective of the present invention is to provide a method for manufacturing an electrochemical device, which comprises a preprocessing step for discharging a battery by applying external power tothe battery before a step for initially charging and activating the battery, thereby removing a by-product layer on a surface of a cathode. The present invention can improve the cycle life of the electrochemical device by comprising the preprocessing step for discharging the battery before the step for initially activating the battery.