"The Uncertainty of Taiwan's Status" is one of the paralogism of some American politicians,whose main purpose is to break up China and make it a legal fact. Some Taiwanese who want to make Taiwan a independent country has been making full use of the choplogic above, especially the leaders of Democratic Progressive Party like Lü Xiulian, who drums up "the Uncertainty of Taiwan's Status " openly after Democratic Progressive Party came to power. According to their logic, the Japanese occupation of Taiwan is based on "Sino-Japanese Treaty of Shimonoseki", but after that no treaty was made to prove the return of Taiwan to China. By Lü's words,"Cairo Declaration" and "Potsdam Declaration" are only declarations but not international treaties and have no legal binding force, hence the problem of Taiwan has no international law resolves yet. This article is to prove from international laws and customs that"Cairo Declaration" and "Potsdam Declaration" are legal treaty by the means of international law. On the contrary, "Sino-Japanese Treaty of Shimonoseki" is thoroughly an unfair treaty and is illegal. In conclusion, " the Uncertainty of Taiwan's Status " is incorrect both in the logic and in the principle of international laws.