Abstract: The energy sector contributed nearly 90% of China’s carbon emissions. Therefore, the multi-dimensional quantitative characterization and analysis of energy flow and efficiency of consumption structure has extremely important significance for the top-level design of carbon neutral path. Energy has both quantitative and qualitative attributes. In this paper, quantitative and qualitative analysis of energy system were combined based on the Sankey diagram. At first, energy and exergy flow of China in 2010 and 2020 were studied and depicted from the perspective of the first law of thermodynamics and the second law of thermodynamics respectively. Moreover, from the perspectives of energy production, processing, conversion and consumption, this paper compares and analyzes the energy flow, energy efficiency and loss of each chain of China’s energy system. It’s also found that China’s energy efficiency has steadily improved with the end-energy efficiency from 74.52% in 2010 reaching 79.04% by 2020. At the same time, China’s energy structure has been transformed to green and lowcarbon. However, during thesame period, China’s energy system efficiency is only 17.44% and 23.91%, respectively, which is far below the energy efficiency. The exergy analysis shows that there is a great loss within the energy system from the energy perspective, including the low-quality utilization of high-quality electric energy and heat energy in the process of electrification, irreversible loss in the process of primary energy combustion, etc. Based on this, the idea of energy efficiency in China from energy perspective to exergy perspectives is put forward under the carbon peaking and carbon neutrality goals and four suggestions are put forward at the same time.