Infrared smoke screen is an important countermeasure method against infrared imaging tracker. The degree of infrared smoke screen shielding on ship target directly affects the tracker’s performance. In this paper, the box counting dimension is used to evaluate the degree. The key problems involved in the computation of Differential Box Counting (DBC) are discussed, which are the range of linear scales and the precise number of boxes, etc, and then the precise computing method for box counting dimension is presented. By the proposed box dimension, the fractal dimension value of the jamming region of ship target image is gotten, which can indicate the degree of infrared smoke screen shielding on ship target and achieve the evaluation of shielding effect.