Distortion is the key index of lithography lens system, but there is no much reports about it. This paper presents a new theory and system about distortion detect, and a new method about how to process the detected data. A new 6-inch dimension, 3 μm resolution large field projection lithography lens was detected with this system and the results is that the distortion of this lithography lens is less than 2.0 μm in all field.