Abstract: Due to the difficulty to do structural calculation and the big quantity of truss beam members in the prefabricated steel frame structure with diagonal bracing，this paper proposed a simplified calculation method by equating steel truss beams with solid I-steel beams according to the principle of equal stiffness，equal height and equal flanges． 5 equivalent beam sections figures were obtained by considering the 4 boundary conditions: simply supported beam subject to uniform loads，simply supported beam subject to point loads at mid-span，cantilever beam subject to uniform loads and cantilever beam subject to point loads at free end． SAP 2000 was applied to build an actual structural calculation model and equivalent ones of a 20-storey prefabricated steel frame structure with diagonal bracing joints． The structural calculation was performed to give a comparison of the key indexes such as mode periods，drift，inter-story drift angle，shear-weight ratio，rigidity-to-gravity ratio and so on． The best match happened between the results achieved in the boundary condition of simply supported beam subject to uniform loads and the actual model． It was proved that this method could be used in integral structure analysis and design for the prefabricated steel frame structure with diagonal bracing and other similar structures．