The cross-section mechanical properties of full-size detachable formwork with steel bar trusses were tested under construction stage and the failure mode as well as the mechanical behavior were analyzed under monotonic load, and the cross-section properties were calculated using load-deflection curve method and ultimate moment method. The analysis results showed that truss model with its chords to be continuous and webs can be used during construction, the error was much large between results of load-deflection curve method and theoretical values, and the reason was that the specimen in failed procress is controlled by deformation, rather than the strength; and the additional deflection was generated by the relative slip between connector and reinforcement; and ultimate moment method were in good agreement with theoretical values. On the base of results, the suggestions were given that the biggest unsupported length under simply supported or continuous beam model.