The invention provides an image brightness acquisition method in external optical compensation. The image brightness acquisition method in external optical compensation is characterized by utilizing atwo-dimensional Gaussian distribution function to perform fitting on the brightness data of each pixel in a first pattern and the brightness data of each pixel in a second pattern respectively, and ignoring a first amplitude deviant Ch and a second amplitude deviant Cb obtained through data fitting to obtain a two-dimensional Gaussian distribution function f(x)H of the brightness of each pixel inthe first pattern and a two-dimensional Gaussian distribution function f(x)B of the brightness of each pixel in the second pattern, so that the brightness value of each pixel in the first pattern andthe brightness value of each pixel in the second pattern which are calculated by f(x)H and f(x)B are in the same level, thus being able to eliminate the brightness difference which is generated whenone image is divided into the first pattern and the second pattern to shoot, so as to enable compensation to be relatively more accurate.