Abstract: Breaking through key part of core technology is a major practical problem and theoretical research topic in China. Proposals for formulating the 14th Five-Year Plan (2021-2025) for National Economic and Social Development and the Long-Range Objectives Through the Year 2035 explicitly states that "Sci-tech self-reliance and self-strengthening as the strategic support for national development, the new system concentrating nationwide effort and resources on key national sci-tech undertakings should be improved in the context of the socialist market economy, and key part of core technology should be strived to make major breakthroughs". However, the academic research on breaking through key part of core technology is still in its infancy, and there are not many researches abroad. The limited literature and reports in China mainly focus on a few topics such as leadership speeches, government role, identification methods, innovation barriers, breakthrough paths, institutional mechanisms and case description. Scholars has made a preliminary study on the concept, characteristics and influencing factors of key part of core technology, but it is far from reaching a consensus. In view of this, we especially explores some basic theoretical issues in this field: concept, connotation, characteristics and breakthrough factors of key part of core technology. Firstly, key part of core technology can be understood from two perspectives: the concept of “or” relationship between “key technology” and “core technology”; “key” represents the degree of importance, “core technology” is the body. Comprehensively reviewing the concepts of key part of core technology in existing literature, we find that some define it through the first perspective and quote the concepts like “core technology” from previous studies, which enlarges or reduces the connotation and extension of key part of core technology and blurs the boundary characters between them; Others make definition from their own interest, but perspectives vary from one to another. Based on this, we believe that key part of core technology is a necessary and insufficient condition of core technology. Combining the related theories of technology gap, the view of sustainable development and holistic approach to national security, we define the concept and explain the connotation of key part of core technology. Then, from the perspective of process, the main characters of key part of core technology are described from four aspects: technical status, research process, breakthrough mechanism and innovation achievements. Finally, the contradiction between a dilemma that one country’s high-end sci-tech development being blocked by other developed countries and that country's domestic security and market demand has triggered a theoretical discussion on breaking through key part of core technology and realizing sci-tech self-reliance and self-strengthening. Taking Industry-University-Research as the body, we analyze seven main breakthrough factors of the key part of core technology from the two dimensions of "breakthrough path" and "assistance channel": Technology suppression cognition and demand orientation, International science technology cooperation and open innovation, Deep integration of Industry-University-Research, Long-term sufficient R&D investment, Leading talents and their teams, Technology chain and industry chain, Government policies and institutional mechanisms. The conclusion can provide a basis of reference for the further research and practice.