1. A method of generating a high dynamic range (HDR) image, applicable to a digital image pickup device for generating an HDR image according to a single digital image, the method comprising: obtaining an original image; performing a gray-scale processing to convert the original image into an original gray-scale pattern, wherein each pixel in the original gray-scale pattern is assigned with a gray-scale value; performing a conversion procedure according to a gradient and a compensation value of the gray-scale value of each pixel in the original gray-scale pattern, and generating a high dynamic gray-scale pattern by using the gradients and the compensation values; and compensating the original image with the high dynamic gray-scale pattern, so as to generate an HDR image.