Since the 1990s, China has made great progress in developing its social security system, achieving the goal of universal coverage for various segments of the population. Nevertheless, it is urgent to improve the quality of coverage, specifically in tackling the problems of inadequate participation and reduced contribution levels in the private sector. This paper examines the changes in social security coverage over the past three decades, focusing on the basic pension insurance for employees of urban enterprises and analyzing it in terms of employment structure. A regression analysis of provincial panel data from 2001 to 2019 indicates a negative correlation between the adequacy rate of social security contributions and the number of employees in private entities and self-employed individuals. Additionally, an empirical analysis using micro-level data from enterprises further confirms lower contribution levels in private enterprises. In light of the present circumstances, it is important to focus on private enterprises, flexible workers, and self-employed individuals as key target groups for expanding social security coverage. Consequently, reforming the social security system should proactively adapt to the requirements of emerging employment formats, striving to achieve comprehensive coverage and enhance the quality of participation across various groups of people.