Absolute poverty will be completely eliminated in China in 2020. However, due to the imbalance between urban and rural development, the differentiation of basic public services between urban and rural areas, and the lack of robust internal motivations in some special groups of people, relative poverty will become a long-lasting problem and a key factor hindering social and economic progress and social stability. Therefore, the study of China's relative poverty will provide a theoretical reference for the establishment of a long-term mechanism to solve the problem of relative poverty in rural China in the post-2020 period. Taking farming households as the research object and 2018 as the time frame of the research, this study systematically analyzed the connotation and characteristics of relative poverty in the post-2020 period and built a measurement and evaluation index system for relative poverty. It also defined the relative poverty line based on a multidimensional perspective and conducted an empirical analysis with Changshou District of Chongqing Municipality as an example. The research generated the following results: 1) The relative poverty index is a useful measure that defines the relative poverty line from a multidimensional perspective of materials, development, ability, and social security. The empirical analysis demonstrates that the relative poverty index is more in line with the actual situation than the single income method in defining the relative poverty line and it can fully meet the needs of reflecting or quantifying actual living conditions of people. 2) Because of the weak support from industry, less opportunities to participate in the cooperative development and skill training of labors, lower level of education, and the fact that more than a third of the relatively poor households are elderly, disabled and seriously ill persons, the relative poverty population are less likely to improve their quality of life, strengthen their capacities, and have access to development. 3) Poverty alleviation policies, economic foundations, social security, and personal conditions are important factors affecting relative poverty. Among them, regional policies are the external causes of relative poverty, economic factors are the key to stimulate the formation of relative poverty, social factors are the basic impetus to prevent the formation of relative poverty, and personal factors are the internal cause of relative poverty.