The present invention discloses a method and a controller for commissioning a wave division multiplexing optical network during capacity expansion. The method includes: calculating, according to a preset rule, initial power of a wavelength service to be added and adding one or more wavelength services according to the initial power; detecting power of each newly-added wavelength service in transmission and separately calculating target power of each newly-added wavelength service according to a detected value and the initial power; calculating end-to-end performance of an existing wavelength service and end-to-end performance of each newly-added wavelength service when each newly-added wavelength service reaches its respective target power; and adjusting the power of each newly-added wavelength service when each newly-added wavelength service reaches its respective target power and the end-to-end performance of the existing wavelength service and the end-to-end performance of each newly-added wavelength service meets their respective lowest threshold requirements.