1. A known face guided imaging method, applied to an electronic imaging device, comprising the steps of: defining at least one scale constraint of a searching window based on a scale of a boundary box of a sensed known face of a previous frame, and said scale constraint is a scale range of said searching window; searching a face horizontally and vertically in a current frame based on said searching window within said scale range; and sending an image patch within said scale range in said current frame to a face detector to perform a face detection.