Human capital, as an important factor of production, directly promotes economic growth. On the other hand, it indirectly promotes economic growth by stimulating the accumulation of physical capital and improving the level of technological innovation. Therefore, human capital is the fundamental reason for economic growth. The key to make human capital play a long-term role is to correctly set the human capital accumulation equation. Different from the existing articles that set human capital accumulation equation from the perspective of human capital investment and physical capital investment, this paper focuses on the marginal output of physical capital. Based on the marginal output of physical capital, this paper makes human capital accumulation subject to a regulation that can completely offset the diminishing effect of physical capital marginal output and keep it at a reasonable level. This way of human capital accumulation is called "regulated" accumulation. The derivation of theoretical model proves that the regulated human capital accumulation, which is based on constant marginal output of physical capital, enables the growth rate of output to be constant and positive. Therefore, this paper finds the source of long-term economic growth from aspect of human capital. As the embodiment of a country's comprehensive strength and core competitiveness, human capital has important strategic significance and great potential in China's sustainable economic development. Regulated human capital accumulation can provide a new driving force for China's economic growth and promote long-term development.