There is provided an optical navigation device including at least one light source, an image sensor and a processing unit. The light source illuminates a work surface in a first brightness value and a second brightness value. The image sensor receives reflected light from the work surface and outputs a first image frame corresponding to the first brightness value and a second image frame corresponding to the second brightness value. The processing unit calculates a differential image of the first image frame and the second image frame and identifies an operating state according to the differential image.