Abstract: In order to provide a nicety gist for Chinese monetary policy, on the basis of fitting the constitution of national debt yield curve with spline, Nelson-Siegel model is used to fit the national debt yield curve.The authors analyzed the reasons why the yield of short-term national debt is higher than the yield of long-term national debt and how the national macroeconomic policy and the microscopic market environment enable the yield of long-term national debt to be higher than the short-term national debt.A conclusion is drawn that the Nelson-Siegel model is more suitable for our national debt yield curve by comparing the models stability.