The implementation of EPC mode in energy-saving reconstruction of existing buildings is an effective market mechanism proved by practice at home and abroad. This paper expounds the development history and market characteristics of energy-saving transformation of existing buildings in China, and analyzes the principle, classification characteristics and framework of EPC mode for energy-saving transformation of existing buildings Under the EPC mode, the operation mechanism of the main body responsibility, operation mode and implementation process of the existing building energy-saving transformation market is analyzed. The development process, practical problems and causes of ESCO under the EPC mode of the existing building energy-saving transformation are analyzed, which is conducive to promoting the practice of the existing building energy-saving transformation EPC mode and the healthy development of the market.