This book points out the legal roots of the alignment of Cross-Strait political relations and the issues of Taiwan's participation in international space, and the Treaty of San Francisco and the "Undetermined Status of Taiwan". Based on an academic standpoint, the book studies the legal theories related to the alignment of Cross-Strait political relations and the issues of Taiwan's participation in international space from the Mainland Chinese perspective. It focuses on the different descriptions and regulations of the alignment of Cross-Strait political relations between the Mainland of China and Taiwan and discusses the status, forms, problems, and prospects of the coexistence of the two sides in the international space. Compared with the policy oaths used in current studies, the book systematically discusses the alignment of Cross-Strait political relations and the issues of Taiwan's participation in international space with a theoretical interpretation. It uses detailed historical materials, especially valuable policy documents and excerpts of speeches cited of the Mainland of China. This book puts forward a series of important propositions, such as the construction of a mechanism for Taiwan's orderly participation in the international space and means of existence of the Taiwan region in the international space.