1. An automatic image capturing system comprising: an image capturing device for focus-shooting a target object; a monitor for displaying a pre-shot image, wherein said pre-shot image includes a pre-shot object image; and an automatic image capturing program for performing a pre-shooting operation on said target object within a first focusing range such that a shooting area and said pre-shot image are displayed on a display zone of said monitor, and discriminating whether a focus-shooting operation is performed according to a relationship between said pre-shot image and said shooting area, wherein if a portion of said pre-shot object image is beyond said shooting area, said automatic image capturing program generates a prompt message, and if said pre-shot object image is located within said shooting area, said automatic image capturing program generates a shooting signal, and in response to said shooting signal, said image capturing device performs said focus-shooting operation on said target object to obtain an object image; wherein if said pre-shot object image is located within said shooting area and said automatic image capturing program discriminates that texts or pictures are included in said pre-shot object image, said image capturing device performs said focus-shooting operation on said target object, and if said pre-shot object image is located within said shooting area but said automatic image capturing program discriminates that no texts or pictures are included in said pre-shot object image, said automatic image capturing program issues an error message; wherein if said pre-shot object image is located within said shooting area, the amount of high-frequency data included in the pre-shot object image is calculated to discriminate whether texts or pictures are included in said pre-shot object image; wherein if the amount of high-frequency data included in said pre-shot object image is larger than or equal to a high-frequency data threshold value, said automatic image capturing program discriminates that said pre-shot object image contains texts or pictures, and if the amount of high-frequency data included in said pre-shot object image is smaller than a high-frequency data threshold value, said automatic image capturing program discriminates that said pre-shot object image contains no texts or pictures and issues said error message; wherein said prompt message and said error message are displayed by a dialog box that is shown on said display zone.