Abstract: A connection structure and a method are provided for being able to make the signal of multi-mode fiber accepted by the single-mode fiber or to make the signal of single-mode fiber accepted by the multi-mode fiber. The present invention uses the core of the cladding of the single-mode fiber corresponding to that of multi-mode fiber in end-to-end relationship or uses the cladding of the single-mode fiber connects with that of multi-mode fiber in side-to-side relationship. Therefore, for users, it will not be necessary to use light-electricity converter between single-mode fiber and multi-mode fiber. In this way, the present invention can help to save the material cost and simplify the arrangement of the transmission line.