Abstract: This paper constructs an economic growth model with population age structure index ,based on the panel data from 31 provinces or municipalities in China for 15 years ,using System GMM methodology to analyze the impact of population age structure on economic growth and using counter-factual methodology to predict the long-term and short-term impact of universal two-child policy on China’s economic growth .According to the analysis, it is found that there is a positive correlation between the proportion of the working-age population and the economic growth, the proportion of the elderly population, the old dependency ratio and the youth dependency ratio are negatively correlated with the economic growth. If China had implemented universal two-child policy in 2000,in the short term ( within 15 years ) ，the average annual growth rate of children's population should have increased from -2.24% to 0.62%, and the total population in 2015 should have increased from 1.342 billion to 1.492 billion, and the average annual proportion of the elderly population should have decreased by 0.52 percentage points, and universal two-child policy should have improved the average annual GDP growth rate by about 0.2045 percentage points through the improvement of the population aging. While the universal two-child policy will have a negative impact on economic growth in the short term by adjusting the population age structure (the decline in the proportion of the working-age population and the increase in youth dependency ratio), the impact of long-term universal two-child policy on economic growth will be positive. Therefore, it is necessary to take policies and measures matching with the universal two-child policy to avoid the negative impact in the short term.

Key words: Universal Two-Child Policy；Economic Growth；System GMM；Counter-Factual Methodology