Objective: To explore the mechanism and influence of population aging on medical insurance expenditure in China and put forward the improvement strategy of medical insurance. Methods: Constructed the structural equation model based on some related indicators in 31 provinces and cities of China from 2005 to 2015.Results: Firstly, the indirect effect of population aging on medical insurance fund is higher than the direct; secondly, medical expenses have the most influence degree to medical insurance fund; thirdly, there are action and reaction between per capita GDP and medical insurance fund , while the reaction is more stronger. Conclusion: The government should construct a multi-level mode of disease prevention and control, further increase the government fiscal subsidies for medical insurance, improve the care coverage, and improve the financing mechanisms to develop the sustainable operation of medical insurance fund in China.