<p>After completing the poverty alleviation relocation work, migrants’ social capital, including social relationships and social networks, has undergone significant changes. Whether these changes will affect migrants’ well-being has become the focus of attention for all sectors of society. This paper empirically investigated the impact of social capital on the subjective well-being of migrants based on survey data from migrants relocated from Liangshan Prefecture, Sichuan Province, China, using ordered Logit regression models. The KHB mediation analysis was used to examine the mediating role of physical and mental health. The study found that, first, social capital, social participation, social networks, and social trust all have a significant positive impact on the subjective well-being of migrants. Second, compared with rural resettlement, agricultural part-time, low-income, and male migrants, social capital has a greater positive impact on the subjective well-being of urban resettlement, non-agricultural employment, high-income, and female migrants. Third, physical health and mental health play a partial mediating role in the influence of social capital on the subjective well-being of migrants, with mediating effects of 30.26% and 18.50%, respectively. This study shows that migrants’ social capital is significantly related to subjective well-being, with physical and mental health playing a mediating role. The government should attach great importance to the role of social participation, social networks, and social trust in enhancing the subjective well-being of migrants, and strengthening the accumulation of migrants’ social capital and maintaining their physical and mental health are the keys to improving the subjective well-being of migrants.</p>