Using the method of intertemporal budget constraint, this paper analyses the long-term stability of national debt, and draws the conclusion that China's national debt system is unstable. This is the result of the requirement of the economic growth and the fact of China's ability to pay for and issue the debts. After analyzing main debtor's requirement of national debts, this paper also estimates the appropriate dimensions of national debts. In its conclusion and using a simplified model, this paper estimates the medium and long-term fiscal income and expenses and makes some simulations for it.