Fairness is not only the internal demand of individual development, but also the core value goal of social development. There are hundreds of millions of migrant population in China, the increase of migrant population of fairness can not only promote its integrated into the society, achieve good social cooperation, also can ensure the stability of social cooperation system. Social security naturally has the characteristics of pursuing fairness, itis important, through the social functions of social security, to improve the equity perception of migrants in order to promote the rational and orderly migration of the population between rural to urban and urban to urban areas. Then, this paper will explore the social security participation of migrant population. At the same time, this paper focuses on whether social security participation affects the socioeconomic equity of the migrants inthe process of population migration. It examines what impact mechanism is involved and asks whether there are differences among the different types of migrants (rural–urban migrants and urban–urban migrants). This series of issues requires in-depth research and exploration.Based on functional theory, institutional theory, and social cognition theory, this paper used the CGSS 2015 for research. First, this paper summarizes the individual characteristics, family characteristics, and social security participation of the migrant population and compares the differences between migrant population and non- migrant population, rural-urban migrants and urban-urban migrants. Meanwhile, this paper analyses the impact of the act of flowing on the social security participation by using propensity score matching (PSM). On this basis, it focuses on testing and researching the effects of social security functions and the objects of social security functions and clarifying the impact of social security participation on the socioeconomic equity of the migrant population and impact mechanisms. At the same time, this article will compare, analyze and explain the situation of different types of migrant population (rural–urban migrants and urban–urban migrants). The main conclusions are as follows: (1) the act of flowing has a significant negative impact on social security participation. Compared with non- migrant population, the probability of migrant population participating in social security is lower, with a decrease of about 9%. At the same time, the probability of rural-urban migrants participating in social security is lower than that of urban-urban migrants, a decrease of about 12%. (2) Social security participation can improve the socioeconomic equity perceptions of migrants by about 14%. Among migrants, the socioeconomic equity perceptions of rural–urban migrants have increased more significantly, by about 22%; but the test results for urban–urban migrants have not been significant. (3) Institutional evaluation plays an intermediary role in the impact of social security participation on and the socioeconomic equity of migrants as a whole and rural–urban migrants in particular. Based on the above conclusions, this article proposes that the government needs to pay full attention to the function of social security to promote social equity, improve and integrate the social security system and eliminate institutional barriers. Attach importance to the system evaluation of the migrant population, improve the awareness of social security; strengthen the equity characteristics of social security and increase socioeconomic equity perceptions of migrants. In the end, the migrant population should be reasonably guided to flow between rural to urban and urban to urban areas.