The rule of law system is the first legal construction project proposed by the Fourth Plenary Session of the 18th CPC Central Committee. Before the Fourth Plenary Session of the Central Committee of the Communist Party of China, no matter the political circles or the academic circles, whether at home or abroad, there was no knowledge about what the rule of law system is, and there was no real discussion about what the rule of law system is. The document of the Fourth Plenary Session of the CPC Central Committee did not make a clear explanation on what is the legal system. By carefully reading the documents of the Fourth Plenary Session of the CPC Central Committee, the Outline of the Construction of a Government Ruled by Law, the Outline of the 13th Five-year Plan, and the Report of the 19th National Congress of the Communist Party of China on the rule of law, we can find that the legal norm system, the implementation system, the supervision system and the guarantee system of the rule of law, and the system of laws and regulations within the party in the paragraph of the “overall goal of ruling the country by law in an all-round way” in the Decision on Ruling the Country by Law in an All-round Way are the legal system as the construction object. The socialist legal system with Chinese characteristics, as the “overall goal of ruling the country by law in an all-round way”, is a legal system composed of these five branches. The other sentences in “detailed description” describe the system attribute and direction, construction path and construction means of the legal system. The definition of the rule of law system as a system composed of five branches is consistent with the general plan of “ruling the country by law and building a socialist country ruled by law”, the general spirit of the Fourth Plenary Session of the CPC Central Committee, and the general goal arrangement in the Decision on Ruling the Country by Law in an All-round Way. The rule of law system in Xi Jinping’s thought is a rule of law system with the essential characteristics of “Chinese characteristics” and “socialism”. The construction of a rule of law system consisting of five branches is the core task of the new stage of China’s rule of law construction, and the main task to improve the level of building a country under the rule of law. China’s “promoting the rule of law in an all-round way” and “building a socialist legal system with Chinese characteristics” are perfect. The major measures of the socialist system with Chinese characteristics are also important measures to promote the modernization of the national governance system and governance capacity. The rule of law system is a brand-new legal knowledge, and the construction of legal system is an unprecedented legal construction project. A clear definition of the constitution of the rule of law system is conducive to delimiting the boundary of the new legal knowledge of the rule of law and clarifying the elements.