The so-called essence, for a certain theory, refers to the most essential thing that can make this theory form, develop and run through it, and at the same time be embodied in the basic viewpoints of this theoretical system. The essence of all theoretical achievements of Marxism in China is seeking truth from facts. First, the thought of seeking truth from facts runs through Mao Zedong Thought, which was founded by the Communist Party of China (CPC) people, with Mao Zedong as the main representative, in the process of leading China's revolution and construction. Many theories, such as the theory of new democratic revolution, the theory of socialist revolution and socialist construction, and the theory of party building, also embody this essence. Second, since the reform and opening up, the Party has adhered to the unity of emancipating the mind and seeking truth from facts, vigorously carried forward the spirit of seeking truth from facts, and consciously liberated ideological understanding from the shackles of outdated concepts, practices and systems, from the wrong and dogmatic understanding of Marxism, from the shackles of subjectivism and metaphysics, and answered a series of major theoretical and practical questions with theoretical innovation based on practice, providing theoretical guidance for the reform and opening up. Thirdly, it is always seeking truth from facts that runs through the theoretical achievements of Marxism in China. Grasping this essence means grasping the historical relationship between the theoretical achievements of Marxism in China and its unified scientific ideological system, and grasping the most essential thing of the theoretical achievements of Marxism in China.