As the population ageing process accelerates, the sustainability of China's health system faces increasingly significant challenges. Therefore, exploring the impact of population ageing on health expenditure is of great importance. Based on data from hospital admission records in Beijing and the China Family Panel Studies (CFPS) data, the Das Gupta decomposition method is adopted to analyze the contribution of population ageing to the growth of health expenditure. The analysis of hospital admission records from 2016 to 2019 reveals that the change in the age structure contributed 22.44% to the growth of hospitalization expenses. CFPS estimates show similar national results. The impact of population ageing on health expenditure is more pronounced among the elderly, with age structure changes accounting for 24.24%, 37.47%, and 31.51% of the variations of hospitalization expense for patients aged 50-64, 65-79, and 80 years old and above, respectively. Improving healthcare financing, promoting healthy ageing, and enhancing system efficiency are recommended.