The demographic dividend is a key driver of China's rapid economic growth since the reform and opening-up in the late 1970s. However, few studies have systematically covered its full cycle—investment, profit, and repayment periods. Five paths of the demographic dividend to promote economic growth are deduced through mathematical reasoning, and the relationship between population policy, the demographic dividend, and long-term economic growth is further examined. A new economic growth accounting method is constructed with the idea of counterfactual simulation, which realizes the path decomposition and overall sum of the economic growth effect of the demographic dividend, and then compares and analyzes the time sequence, path and effect intensity of the demographic dividend under different policy scenarios. The study finds that population policy has a significant impact on long-term economic growth in the dimensions of economic aggregate, growth trend and mode. The channel through which population policy affects economic growth is the demographic dividend, and it exerts its influence by changing the timing of the formation of the demographic dividend, the path of its action, and the strength of its effect. The economic growth effect of population policy is long-term and persistent, and the economic growth paths under different policies may converge in the short run but still diverge in the long run.