In the requirement of the “double carbon” goal, China has confronted the lack of a driving force for the low-carbon transformation of socioeconomic development, which requires a comprehensive law and strategy support system for supporting a circular economy. In this study, a framework associated with a multi-level quantitative index system associated with legislative support for a circular economy (MILC) through the entropy TOPSIS method was developed. It can not only reflect the legal support for a circular economy in four areas based on the “3R” principle, but also respond to the process of environmental law improvement dynamically. The legislative support effect can be applied and analyzed in China’s circular economy for the period from 2009 to 2022, which can respond to the process of legal improvement on the environment dynamically. The obtained results show that China’s circular economy and its rule-of-law guarantee system are still facing many challenges, such as the low systematic degree of legalization for industrial development, unmatched supporting legal system, and backward concepts and consciousness of circular economy legalization. Various suggestions according to the identification of the importance of a legal support system for the circular economy were obtained, which can not only encourage a reduction in resource consumption and carbon reduction from the view of systematic legalization, but also promote socioeconomic transformation to match the goal of “double carbon”.