Taking a civil air defense tunnel project in Jiangsu Province as the background, a construction technology with the large span steel truss formwork for chamber excavation has been developed by study and integration of the steel formwork trolley, load-bearing scaffold, steel truss and steel formwork through field use and comprehensive comparison of safety, economy and maneuverability, the construction methods. Technical study and application of the system in underground chamber excavation is discussed, with emphasis on the process principle, formwork design and stress analysis, engineering application, etc. The construction method can improve efficiency, save cost and improve construction quality.