Abstract: From the perspective of modern geopolitics, the marine sovereignty directly sustains the rise and fall of the country. As the most basic demarcation standard, the island is the most important territorial symbol and sovereign symbol in the ocean. Therefore, the island naturally easier to become the fuse of sea power disputes, boundary-point island or nearby waters even more so. At present, the situation in South China Sea , the East China Sea as well as the Yellow Sea are is not optimistic, and the sea power problem in China is facing a severe test. Therefore, how to examine the strategic positioning of these border islands will become a major subject of our country. This paper attempts to use the corresponding concept in land border system - "boundary", taking Hainan Island and Jeju Island as an example, and analyzes the geopolitical situation of the island at the boundary point in marine frontier. The purpose of this study is to explore a new model of sovereignty over the territorial sovereignty over the sea, which can play a certain enlightening effect on the protection of marine rights for our country.