Disclosed is an optical navigation system, which comprises: a main control circuit; an activating device, coupled to the main control circuit; and an optical sensor, coupled to the main control circuit and the activating device. The main control circuit controls the activating device to transit from a first standby mode to a first active mode when the optical sensor enters a second standby mode. The activating device generates an activating command to transit the optical sensor from the second standby mode to a second activate mode corresponding to a specific action, when the activating device is in the first active mode. By this way, the optical sensor can be activated via another device rather than the optical data sensed by itself, thus the power consumption can be reduced.