The invention relates to an image forming method. The method comprises the following steps of: inputting a color image; converting the color image into a first gray scale image; reducing the first brightness range of the first gray scale image into a second brightness range; generating a first gray scale gradient image corresponding to the space distribution of the first gray scale image according to the first brightness range and the second brightness range; superimposing the first gray scale image with the second brightness range and the gray scale gradient image to generate a second gray scale image; generating a translation image according to the second gray scale image and the color image; and forming a stereoscopic image according to the color image and the translation image.