A system for assisting fixed-focus image-capturing device to focus, which includes: an image-capturing device with fixed-focus function; a platform for placing objects; a movable structure connected to the image-capturing device or the platform, for adjusting the distance between the image-capturing device and the platform; a notifying unit, which generates a physical quantity to notify a user; and a control unit in communication with the image-capturing device and the notifying unit, for determining the distance between the image-capturing device and the platform, and when the distance is equal to a best-shot distance, sending a signal to the notifying unit to generate the physical quantity. The invention also discloses a method for assisting fixed-focus image-capturing device to focus.