In a method for detection of vital sign signal, a variation detection module is configured to determine whether there is a living body in an image captured by an image capture module, next, a beam direction of a vital sign sensing system is directed toward the living body to detect vital sign signal of the living body, and a compute module is configured to compute eigenvalue and confidence of the vital sign signal so as to reconfirm whether the determined living body in the image is a living body having vital signs.
