Abstract: After the function of existing large-span steel structure buildings have changed, in order to meet the requirements of office buildings for roof waterproofing, fire and smoke exhaust, lighting, heat insulation, comfort, energy saving, environmental protection and aesthetics, their roof need to be reconstructed. Conventional reconstruction methods need to reconstruct item by item so as to meet various index requirements. It make the construction process of the reconstruction method very complicated, but the reconstruction effect is poor and often difficult to achieve the expected effect. Besides, it may also bring other new problems. In order to solve this technical problem, the paper proposed technology adopts a combined design method: the middle part adopts ETFE air pillow type membrane structure (part of it is set as fuse membrane, and the number and location of fuse membrane are designed according to the requirements of fire protection smoke), Tthe original aluminum veneer roof is retained at the edge of the roof, and the area between the ETFE air pillow and the aluminum veneer is the newly added aluminum-magnesium-manganese metal roof. After the reconstruction is completed, the existing large-span steel structure public buildings can meet the requirements of various indicators. At the same time, compared with conventional transformation methods, this technology has the advantages of simple structure, simple construction and good transformation effect.