Abstract: An optimizing idea was produced for the design of the top beam in electrostatic precipitator casing: the insulation layer and the original short-span top beam of casing could constitute a long-span truss beam system，then the overall cross section of the truss beam was heightened，which enabled its bending bearing capacity to be much greater than the original short-span beam，thus，the span of beam system could be enlarged． The original double short-span beams could be transformed into a long-span truss beam system and the original separating wall could be dismantled． Consequently，the steel consumption was reduced． By the method of one-dimensional beam-link structural analysis and the finite element method of shell element，the design and internal force evaluation method for the long-span truss beam system were investigated． Besides，the load bearing performance of bottom chord with box section was emphatically investigated by the shell finite element method． The preliminary design method of constituting members，the optimum configuration and the internal force evaluation method for the beam system were proposed． For the bottom chord，the load bearing evaluation method of end face of supporting stiffener，the sensitive local buckling mode and the most unfavorable initial geometrical imperfection mode were obtained． Besides，the optimizing suggestion was also produced for the size design of transversal stiffener and the height position of axial stiffener．