Space primary mirror splicing technology is one of the development trends of the future cosmic observation. Compared with the traditional space primary mirror, the splicing primary mirror has the advantages of large aperture, breaking the size limitation of carrier fairing, high resolution, light and active adjustment, etc. Based on the literature and research project at home and abroad and the different forms of splicing structures, the deployable splicing space primary mirrors are divided into on-orbit deployed type, on-orbit splicing type and on-orbit distributed typeo. And the concepts, characteristics and splicing structures of these three forms of space splicing mirror are described, respectively. According to the three classification forms, the research backgrounds and research contents of present known splicing primary mirror projects are introduced. This paper summarizes the key technologies involved in the splicing structure of the primary mirror, and summarizes the two research difficulties which are the splicing precision and the face precision as the starting point.