The blue sun exposure method is also called iron cyanic acid (iron-Prussian blue) printing method. Its chemical ingredients and treatment methods are very simple and have many aspects. This basic printing method can produce a white image and shadow area with blue intermediate tones. Other colors can also be produced, including a blue image on the white background, and can be made on the cloth of the photo paper or the cloth washed with a negative or transparent positive film, or a photo is made on other materials.