The biggest difference between offset printing and other printing is that it uses fountain solution in addition to ink in the printing process, which is determined by the structure and nature of flat packaging layout. The printing plate used in lithography is usually a front-coated photosensitive plate, which is called PS plate. The graphic basis and blank basis of this plate are almost on the same plane (the graphic basis is an exposed diazo photosensitive resin layer, and the base surface of the high journal is about 4 microns, and the blank basis is an aluminum oxide film, and the base surface of the high journal is about 0.2 ~ 1 micron). In the printing process, water is supplied to the whole printing plate first, so that the white part of the page width absorbs water to form a water film. Then, ink is supplied to the printing plate. At this time, because the existing water film in the blank part no longer absorbs ink, the graphic part resists water and absorbs ink lipophilic, and then the graphic part transfers ink to the printing piece for printing through the rubber drum. Therefore, fountain solution plays a very important role in lithography.