Abstract: In high energy X-ray radiography system , CsI convertor is usually used to convert the X-ray image into a visible image which can be recorded by other high sensitive image system . The scattering light from the inner side of the convertor can reduce the image contrast and the spacial resolution will degrade. In the paper ,one evaluation method about the influence on the image contrast is set up and some calculated results are given to show this . The tests have proved that the process on the back side of the convertor is the most important thing and that the scattering coefficient of the back side must be reduced to a very small level to keep the image contrast．