A method for adjusting grayscale values of an image includes: for each pixel in the image, calculating a first mapping value for a grayscale value of the pixel in the image through a histogram equalization mapping function; calculating a second mapping value by subtracting a first preset value from the grayscale value of the pixel through the histogram equalization mapping function, and when the result exceeds a preset grayscale range, selecting a minimum value of the preset grayscale range for calculating the second mapping value; calculating a third mapping value by adding a second preset value to the grayscale value of the pixel through the histogram equalization mapping function and when the result exceeds the preset grayscale range, selecting a maximum value of the preset grayscale range; and calculating an output grayscale value of the pixel based on the first, second, and third mapping value and respective weights.