In the offset printing industry, ink quantity plays an important role. At the present time, ink evaluation usually depends on human naked eyes. Although the method is easily to carry out, to the same original image, the results usually differ from the practical ones. In the research, processed gray prepress digital images to be input into plate making device are considered, with image block processing theory, according to parameters such as screening lines and ink density and so on, the ink quantity of the original image on the printed sheet is figured out. Similarly, batch printed sheets ink quantity could be calculated. The experiment shows that the ink quantity reaches rather high exactitude, approaches factual ink quantity in the printing workroom with fairly high precision.