Abstract: Scientifically measuring the level of energy security during energy transition and identifying its dynamic evolution characteristics and major constraints are of great significance for safeguarding China’s energy security and advancing the construction of a new-type energy system. From the perspective of energy transition, a four-dimensional energy security evaluation index system covering energy supply security, energy economic security, energy structure security and energy environmental security is established. The range standardization method, combined weighting method and TOPSIS model are adopted to measure China’s energy security level from 2005 to 2024, and its evolution characteristics, dimensional differences and primary constraints are revealed. The results show that the comprehensive level of China’s energy security presents an overall fluctuating upward trend, with staged evolutionary features including fluctuating improvement in the early stage, steady growth in the middle stage and continuous optimization in the late stage. Among the four dimensions, energy structure security and environmental security have improved rapidly and become the main drivers for the growth of overall energy security; energy economic security has generally improved but experienced fluctuations in recent years; energy supply security remains under long-term pressure and deserves focused attention. Targeting the major constraints on current energy security, it is necessary to promote the coordinated improvement of energy supply assurance, energy economic efficiency and the operational resilience of energy systems, so as to comprehensively raise China’s comprehensive energy security level and achieve the goal of constructing a new-type energy system in China.