Abstract: The identification of ink is a difficult question in criminal case. In this paper how to apply visible-ultraviolet spectrophotometer into the identification of ink was discussed. There are two maximum absorption top for the blue-black ink in the visible-and ultraviolet region. The wave-length are 288nm and 610am. Tge ratio of their absorption value changes with the type of ink, but doesn't change with the concentration and product date of ink.