The carbon peak and carbon neutrality policy has a significant impact on the development of China's construction industry. By analyzing the new requirements for the renovation of existing buildings,a technological path for low-carbon renovation of existing buildings is proposed, following the two major low-carbon operation paths of energy conservation and energy structure adjustment. Through the construction of an energy carbon management platform, the application of building energy-saving technology, and the utilization of renewable energy, existing buildings are transformed into smart buildings, and digital technology is applied to promote building carbon reduction. Research shows that by renovating existing buildings through the above path,the comprehensive energy-saving rate of the central air conditioning cold and heat sources can reach 25%. By renovating the air conditioning units and air conditioning terminals, the annual carbon emissions of buildings can be reduced by 493 400 t;The elevator system can save 11 000 kW·h of electricity and reduce 9 727.3 t carbon emissions annually by installing a variable frequency regenerative energy feedback device. The feasibility and significant carbon emission reduction effect of this technology path provide technical reference for the low-carbon and energy-saving transformation of existing buildings. The "carbon peak and carbon neutrality" policy has an important impact on the development of China`s construction industry. This article proposes a technical path for low-carbon retrofitting of existing buildings by analyzing the new requirements for retrofitting existing buildings. It follows two major low-carbon operation paths for energy conservation and energy structure adjustment, and transforms existing buildings into smart buildings through three methods: the construction of energy-carbon management platforms, the application of building energy-saving technologies, and the utilization of renewable energy. Digital technology is used to promote carbon reduction in buildings.