1. An automatic exposure control method, used for determining a exposure compensation value of an image, comprising: establishing a coordinate system, wherein a plurality of sub-ranges with a plurality of different corresponding exposure compensation values in the coordinate system; setting a bright region threshold value and a dark region threshold value; calculating a ratio of the pixel count in an image, wherein the luminance of each pixel is higher than the bright region threshold value over the total image pixel count, for obtaining a bright area ratio; calculating the ratio of the pixel count in an image, wherein the luminance of each pixel is lower than the dark region threshold value over the total image pixel count, for obtaining a dark area ratio; determining the coordinate positions of the image in the coordinate system according to the bright area ratio and the dark area ratio; and compensating a exposure value of the image with the corresponding exposure compensation value of the sub-range where the image is located when the position of image in the coordinate system is fallen inside one of the sub-ranges.