A method for window adjustment based on simplified focus model was proposed, which can be used in autofocus process of digital camera. Firstly, the field angles of initial focus window boundary points were calculated with initial positions of those points and initial image distance. After the lens moved, the field angles and new image distance were used to calculate the new positions of focus window boundary points. Thus the focus window was adjusted to follow the target. Experimental results show that the proposed method can estimate the drift caused by imaging magnification changing on condition that the object distance is unknown. Compared with stationary focus window, the proposed method can avoid interference of background information, thus making the evaluation function unimodal. Besides, the proposed method is independent of the defocus blur for its calculation process has nothing to do with the image content itself.