Concave lens imaging study of theory and the formation of a shadow principle, can make use of shadow to draw concave lens focal length measurement. According to the concave lens focal length f of the spindle on a complex point A of the imaging light path map, if so, concave lens像距virtual and physical distance, into the formula, will be able to calculate the focal length lens. The use of double shadow law, can be indirectly measured virtual像距concave lens.