A known face guided imaging method applied for an electronic imaging device uses the scale of a known face sensed in a previous frame to define a scale range of a searching window, which adds a scale constraint into the searching window so as to search a current frame horizontally and vertically in the searching window to perform a face searching loop for a face. The invention simply sends a corresponding image patch within the scale range to a face detector for the face detection, and the searching space is constrained in a small group of scale ranges without the need of detecting the whole image of various different scales within the scale range, and thus the invention effectively reduces the huge quantity of computing values required for the detection process and greatly enhances the speed and efficiency of the face detection.