Based on permanent income theory, in terms of demand, this paper constructs a model which can embrace main explanations on China international trade surplus; includes exchange rate viewpoint, consumption viewpoint, resource viewpoint and invest-saving gap viewpoint. Empirical tests show that insufficiency of expected income is the origin of trade surplus. Exchange rate’s impact can’t be ascertained. Insufficiency of investing chance and defect of financial system enlarged the surplus. At the end, the author suggests improve social security system and legal institutions, curbing inflation, and keep exchange rate smooth and steady.