Abstract: The building sector is a key sector to achieve the double carbon target, and under the guidance of the double carbon target, the building energy system needs to make innovation. Therefore, the development tasks of building energy system are discussed in depth in this paper, and the development direction of building energy system facing the dual carbon target is proposed. The traditional building energy system is mainly to meet the basic energy needs of the building itself, such as cold, heat and electricity. Under the dual carbon target, there needs to be a shift from energy efficient buildings to new targets for low-carbon buildings. The building energy system needs to make changes in reducing the energy demand of the building body, fully electrifying the building energy system, improving the energy efficiency level of the building energy system, realizing flexible adjustment and becoming an adjustable load of the energy system with flexible adjustment ability. It needs to shift from being a simple consumer of energy systems to a complex that integrates energy production, consumption, regulation and storage. To build low carbon building energy system as the goal, the trend of research of building energy systems is discussed: the need to further meet the demand of the building energy to make a full understanding of the building energy requirements, e.g. the energy requirements for building thermal environment is a key section for building energy consumption and energy conservation, need further integration architecture and traffic, electricity and other fields, from the monomer building to multiple scales, such as architecture, urban construction as the carrier to build new energy system between urban and rural areas. The current research provides a useful reference for building energy system to realize its own role transformation and accelerate the realization of energy system reform under the dual carbon target.