1947, in the Map of South China Sea Islands compiled and published by the local regional department of the Ministry of Interior of China at that time, a line composed of 1 1 segment was plotted with undetermined national boundaries. After the founding of New China, the map approved for publication by the relevant government departments also marked such a line at the same position, except that the intermittent line of 1 1 was changed into nine intermittent lines. This line is usually called the traditional boundary line, and it is also called the "U" line because of its "U" shape. The "U" line was proposed and approved by the government department of China, and plotted on the official map published by China. It should be regarded as a reflection of a certain position of the government of China, and it can also be regarded as the proposition of the government of China. Therefore, it is of great significance to correctly interpret the legal meaning of this line. In China academic circles, there are many different opinions and explanations about the legal meaning of the "U" line. According to the information I have seen, there are four main propositions and explanations: 1) The national boundary says that this line delineates the territorial scope of China in the South China Sea, and the islands, reefs, beaches, sand and sea areas within the line belong to the territory of China, and China enjoys sovereignty over it; Offline areas belong to other countries or the high seas; 2) Historical waters line, which holds that China enjoys historic rights to the islands, reefs, beaches, sands and sea areas within the line, and all the sea areas within the line are historical waters of China. 3) The historical rights line says that it marks the historical ownership of China, including the sovereignty over all the islands, reefs, beaches and sands in the line, as well as the natural resources in the water inside the line and the sea area outside the seabed. At the same time, it recognizes the freedom of other countries to sail, fly over and lay submarine cables and pipelines in this sea area. In other words, while claiming that the islands, reefs, beaches and sands within the line belong to China, this view regards the waters outside the internal waters as China's exclusive economic zone and continental shelf. 4) The island ownership line or island range line means that the islands within the line and their adjacent waters are part of the territory of China and are under the jurisdiction and control of China. All these claims and explanations should be considered reasonable, not all of them are groundless. However, it is also obvious that only one kind of proposition and explanation can best meet the requirements of objective facts and international law of the sea, and only such proposition and explanation can better safeguard and safeguard China's rights and interests in the South China Sea and help solve the South China Sea issue.