Abstract: Given the shortcomings of existing middle-income group definitions, marked by a lack of theoretical foundation, mechanical operations, and low result accuracy, the paper proposes a refined definition based on vulnerability theory, characterizing the middle-income group as economically stable and non-vulnerable. And then the paper employs vulnerability estimation and kernel density methods to establish upper and lower limits for vulnerability within this group based on the China Family Panel Studies (CFPS), then estimates the proportion of the middle-income group in China, analyzes the micro driving factors of family entry and exit from the middle-income group and judges the evolution trend of the proportion of the middle-income group. The results show that: Firstly, the definition standard of the middle-income group in China can be set as 1% and 10% of the vulnerability corresponding to the family per capita persistent income, and the corresponding proportions in 2014 and 2016 are 29.82% and 32.71%, respectively. There are obvious differences in urban-rural and regional distribution, and urban and eastern regions have tended to form an olive-shaped distribution pattern. Secondly, factors such as human capital, occupation, family burden, and income structure play crucial roles in propelling families into the middle-income group. To meet the 2035 target of “achieving more significant substantive progress in common prosperity”, the per capita persistent income growth rates for the high-income and low-income groups should be a minimum of 3% and 5%, respectively, from 2016 to 2035.

Key words: Proportion of Middle-Income Group, Vulnerability, Definition Standards, Driving Factors