In recent years, light steel prefabricated building have received extensive attention and research due to their simple structure, fast and convenient construction, high environmental benefits and other advantages, and have been widely used in engineering practice.It has great application prospects. The components of light steel prefabricated building are not suitable for secondary damage, but the construction caused by the traditional HVAC design method damages the building structure. Therefore, the traditional HVAC design method is no longer applicable to light steel prefabricated building. At present, the research and application on the integrated development of HVAC and light steel prefabricated building are relatively few. In order to fill the gap of HVAC design method for light steel prefabricated building, a new integrated design method of HVAC system and light steel prefabricated building was proposed in this study. 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 multi split system. The research results show that the new integrated design method of multi split system and light steel prefabricated building can make up for the shortcomings of the traditional HVAC design method applied to light steel prefabricated building. And the method 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.