The purpose of this study is to understand how and why of the exclusion of poor rural immigrants to Tehran in the labor market. In this research, the basic theory method is used, which is a qualitative method. The study population is the heads of urban poor households living in districts 17, 18 and 19 of Tehran, which was selected by purposive sampling and continued until theoretical saturation. After the coding process, the research findings based on the paradigm model of the research are divided into five axes: causal conditions, intervening conditions, context, strategies and consequences, and the nuclear category was obtained as social exclusion from the formal labor market. In this article, the causal conditions are discussed. According to the participants, these conditions include four main categories (weakness of economic capital, weakness of human capital, unfavorable economic conditions and unfavorable working environment conditions). The research findings show that many poor people living in the study areas, who had a rural origin, have experienced all kinds of exclusion in the formal labor market after migrating to Tehran. Meanwhile, unfavorable economic conditions such as economic sanctions, the spread of Coronavirus, mechanization of production and provision of services and the presence of foreign nationals have affected the vulnerability and exclusion of people from the formal labor market. Also, unfavorable working conditions such as low income and lack of social support and physical and psychological harm and exploitation have led to their exclusion from the labor market.