A novel system used to obtain a square focus spot has been developed, which is composed of a pair of segmented wadge arrays that are mutually perpendicular. The features of this system are that, the size of the focus spot can be varied continuously within a certain extent,the shape of the focus spot may not be influenced by the shape of the cross section of the incident beam, and the intensity distribution of the focus spot is of high uniformity and is influenced very weakly by the intensity distribution of the incident beam. In this paper, detailed theoretical analyses and numerical calculations for this system are presented.