In recent years, lightweight steel prefabricated buildings have been widely used in engineering practice due to their advantages of convenient construction and high environmental benefits, and have great application prospects. However, due to the subsequent construction caused by the traditional heating, ventilating and air conditioning (HVAC) design method that may damage building components, the traditional HVAC design method is no longer applicable to lightweight steel prefabricated buildings. At present, there is relatively little research and application on the integration of HVAC and light steel prefabricated buildings. To fill the gap in this field, a new integrated design method of HVAC system IB-VRV (taking variable refrigerant volume air conditioner as an example) and light steel prefabricated building (variable refrigerant volume air conditioner integrating with buildings) was proposed. And the difference between the HVAC design method applicable to traditional buildings and the HVAC design method applicable to light steel prefabricated building were explored. According to the difference between the two HVAC design methods, on the basis of the existing traditional HVAC design concept, three principles of integration of HVAC system and light steel prefabricated building were summarized. These three fusion principles were structural safety priority, non identical life cycle, and functional area elasticity. Based on the above three principles, a new integration mode of HVAC system and light steel prefabricated building was proposed, and analysis was carried out with the case of variable refrigerant volume (VRV) system. The research results show that IB-VRV can make up for the shortcomings of the traditional HVAC design method applied to light steel prefabricated building. And IB-VRV can give full play to the advantages of light steel prefabricated building, such as light deadweight, simple structure, small components that can be concealed. This work opens up a new idea and lays a theoretical foundation for the integrated design of HVAC systems and other types of prefabricated building.