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; building a label mask with the same scale constraint as said current frame, and labeling an area corresponding to said boundary box of sensed known face in said previous frame and other area corresponding to the remaining unknown faces with different values; and if the center of said searching window falls in said area corresponding to said boundary box labeled as having said sensed known face during the face detection, then said scale constraint of said sensed known face will be applied; and when there is only one sensed known face in said current frame, said scale range of said searching window is defined as: 
 R=[x /( s ) 2 , x *( s ) 2 ], wherein x=l/b, and l is the number of pixels of the width of said boundary box of said sensed known face in said previous frame; b is the number of pixels of the width of said searching window with a minimum scale; and s is a scale factor.