Since the 1970s, great progress has been made in research on the spatio-temporal pattern of population ageing at multiple spatial scales by both domestic and foreign scholars. However, the analysis of the process mechanism of regional ageing is still insufficient. Based on the county-scale data in 2000 and 2010 censuses, the spatio-temporal pattern of population ageing is statistically analyzed. Moreover, the Structural Equation Model (or SEM) is developed to explore the path relationship between various variables. Consequently, the influencing mechanism of regional ageing process is thoroughly revealed. The main results are as follows. Firstly, the line from Baotou City of Inner Mongolia autonomous region to Tengchong County of Yunnan province (or BT Line) is a geographical demarcation of regional ageing disparities in China. Southeast of the BT Line witnesses contiguous areas of higher ageing counties which interspersed with lower ageing counties, whereas northwest of the BT Line is a continuous area of lower ageing counties with just one middle-level ageing belt located in the north margin. Secondly, the fitting results of SEM interpret the influencing mechanism of regional ageing disparities in China. Thirdly, life expectancy, fertility and migration are three endogenous factors of regional ageing process. Thereinto, life expectancy has significantly positive effect, while fertility and migration have significantly negative effects on regional ageing. Lastly, geographical environment has significant direct and indirect effects on regional ageing in China. In general, the income increase, social progress, and the improvement of living conditions and natural environmental quality would enhance regional ageing level, otherwise the improvement of urbanization level would decrease regional ageing level.