2021 is the first year of the “14th Five-Year Plan” period，and it is also a crucial year to consolidate the achievements of poverty alleviation after the completion of a well-off society in all respects. Social security is a critical area of people’s livelihood and poverty alleviation. The realistic level of Beijing’s social security system is an important indicator of maintaining social equity and reflecting the civilization of the capital’s social system. This article uses data from the 2015-2020 national and provincial statistical yearbooks to investigate and evaluate the construction achievement of Beijing’s social security system from the aspects of social insurance coverage，actual expenditures on social assistance，and social welfare coverage. The study found that Beijing’s social pension insurance has a high coverage rate and relatively good fund operation and sustainability；urban and rural areas have a relatively high level of social assistance，but the system standards need to be further adjusted；social welfare and development coefficients have steadily increased. The integration process of the fragmented social security system reflects the process of gradual pursuit of institutional fairness. In the new era of social security system construction，Beijing needs to increase the construction of the bottom social safety net，balance the relationship between fairness and efficiency，and improve the multi-level social security system.