Abstract: In this paper, the author points out that there are some "blanks" in the map projection classification systems, and it is possible as well as necessary to fill them. Taking the poly-cylindrical projection as an example, the author depicts the principle of construction. The process of the establishment of poly-cylindrical projection is explained with three examples. The author believes that the supplement of this kind of map projection will contribute to the improvement of the classification systems and the teaching activities of this subject as well.