In order to promote the key work of carbon reduction of small or low energy consumption buildings with high quality and further optimize the energy management of smart buildings, a smart building demand response strategy for flexible load and carbon trading mechanism is proposed to optimize the demand response of smart buildings and clean energy use. Firstly, an intelligent building power-heat energy system containing distributed energy is constructed, and flexible loads such as transferable load, reducable load and transferable load are introduced into the system, and dynamic energy allocation is carried out according to the energy demand of building users. Secondly, considering the stepped carbon trading mechanism, the building carbon emission cost is optimized by adjusting the energy output, the building carbon emission is improved, the energy structure is clean and the transformation is promoted, and the economic and environmental benefits of the system are optimized. Through the comparative analysis of three scenarios, the introduction of flexible load and carbon trading mechanism in the energy system of intelligent buildings can give full play to the interaction and plasticity of the energy structure of intelligent buildings, effectively promote the consumption of clean energy, and realize the low-carbon economic operation of the energy system.