An optical navigation system comprising a control circuit and an optical navigation device. The optical navigation device comprises: an image sensor, configured to generate a plurality of image frames; and a motion reporting device, configured to report a first motion of the optical navigation device to the control circuit at a first time after a reference time, and configured to report a second motion to the control circuit at a second time after the first time. The first motion and the second motion are calculated according to the image frames. The control circuit calculates a first scaled motion according to the first motion, the second motion, a first time difference between the first time and the reference time, and a second time difference between the first time and the second time.