1. A motion detection method for an optical navigation sensor, comprising steps of: capturing an image frame associated with the optical navigation sensor; distinguishing a qualified image frame from an unqualified image frame according to a quality prerequisite; providing a first counter to record incidence of qualified image frames, and a second counter to record incidence of unqualified image frames; when a first condition is met, proceeding motion detection and returning to the capturing step; and when a second condition is met, skipping the image frame and returning to the capturing step.