Abstract: Through the bearing capacity test and finite element analysis of five nest light steel truss beams of different connection types, this paper made a study on the stress characteristics, failure mode and ultimate bearing capacity of nest light steel truss beam under vertical loads.The main factors which affect the flexural rigidity and ultimate bearing capacity of truss beam were also discussed.The result shows that the failure in truss beam mainly occurred at top chord near support or at mid-span due to the flexural or compression bending;the failure position will be changed by decreasing the distance between the first web member and support or using diagonal braces at the truss ends, but their effect on bearing capacity is not obvious;the ultimate bearing capacity of truss beam will be obviously increased after increasing cross section area of the top chord, but the change of bending stiffness of beams is little;the error of deflection simulation analysis is large with node rigid assumption, while results between simulation analysis and experimental values are more close with node semi-rigid assumption;the design of truss beam is mainly controlled by its mid-span deflection, the normal service bearing capacity of the truss beam can only reach 33~45 percent of ultimate bearing capacity.