A method and system for adjusting a gray-scale level of an LCD device. The method includes: checking a default gray-scale value A; comparing default brightness value YA with a target brightness value YX; inputting a trial gray-scale value B; and outputting a target gray-scale value X. The method employs a preselected Similarity of Triangles technique. The system employs a image-capturing unit determining screen brightness and a control unit using a preselected Similarity of Triangles technique to adjust the gray-scale level of a connected display device, reducing a single-screen-capture gray-scale level adjustment interval to about 1.8 to 2.1 seconds.