Abstract: The demographic factor is a non-negligible factor affecting long-term capital flow. At present, China is not only facing the gross problem of the total population about to peak and in the long term possibly shrinking sharply, but also the structural problem of declining birthrate and aging. The rapid aging of population will pull down China's interest rate in the equilibrium state, i.e., the natural interest rate. When constructinga general equilibrium theoreticalmodel to depict the existence and the operation mechanism of the natural interest rate channel, it is found that the changes of population structure can affect the relative changes of the natural interest rate in the equilibrium state, thereby affecting the long-term international capital flow,that is, the natural interest rate is an important channel through which population changes affect international capital flow. With the rapid increasing of China's population aging, the labor force has changed from a relative surplus to a relative scarcity, which will affect the labor-capital input ratio of production activities, thereby driving the labor rate of wages to rise, and the natural interest rate to stabilize after a rapid decline. In the open economy, China will gradually become a net capital outflow economy and face the dilemma of net capital outflow. The impact of population changes on the economic and social development is tremendous and is hardly to reverse. Therefore, it is necessary for China to speed up the construction of a fertility support system to increase the fertility rate, promote the policy of delaying retirement, and make a good reserve of monetary policy tools.

Key words: demographic structure, aging, natural interest rate, international capital flow