Abstract: Cooperative participation is one of the important ways to increase farmers’ social capital accumulation, to enhance the executive ability of the rural poor, and to alleviate relative poverty. Based on the feasibility theory and a micro-survey data of farmers in Henan Province, this paper constructs a multi-dimensional relative poverty index system, analyzes farmers’ participation rate of professional cooperatives and the relative poverty level of farmers, and discusses the alleviation effects of cooperative participation on farmers’ relative poverty and its mechanism by the binary Probit model and the intermediary effect model. Results show that farmers’ participation rate of professional cooperatives in sample areas is low, only 17.6% and 63.6% of farmers are deprived in three or more relative poverty indicators and the relative poverty level is relatively high. Cooperative participation has a significant positive effect on alleviating the relative poverty of farmers. Controlling other variables, the probability of relative poverty of farmers’ participation in professional cooperatives will be reduced by 22.9%. After the robustness test and considering the endogeneity, the conclusion is still valid. Cooperative participation can alleviate farmers’ relative poverty by increasing social capital accumulation, while social network and social trust are the main transmission mechanism. Simultaneously, age, gender, political status of the household and family members’ working experiences have significant impacts on the relative poverty alleviation of farmers. Therefore, to alleviate relative poverty and to achieve common prosperity, this paper provides the following suggestions: strengthening and completing the construction of rural cooperative organizations, enhancing farmers’ confidence in poverty reduction through cooperative organizations, and giving a full play to the poverty-benefiting function of rural cooperative organizations.