Abstract: The paper describes the concept, classification and current situation of U.S. public debt, defines the meaning of public debt sustainability, and then by comprehensively using multiple methods, including comparing the rate of economic growth and public debt interest rates, specific reference value determination method, the inter-temporal budget constraint test and fiscal reaction function test, the paper does empirical study on the sustainability of the public debt of the United States. The conclusion is that the U.S. public debt is not sustainable.