Abstract: An automatic focusing method and apparatus comprise the following steps: acquiring a target image that has been divided into blocks; acquiring the definition of each block, respectively; acquiring normalized central coordinates and a normalized size of a region of interest on the target image; respectively calculating a full width at half maximum coefficient in the horizontal direction and the vertical direction according to the normalized size; calculating a weight value of each block using a two-dimensional discrete Gaussian function according to the normalized central coordinates and the full width at half maximum coefficient; calculating a normalized overall definition of the target image according to the weight value and definition of each block; and focusing according to the normalized overall definition. The method and apparatus can automatically calculate a mask of the region of interest, thereby avoiding the occupying of storage space required when storing ROI mask data.