Abstract: In order to solve the disadvantages of modeling difficulty and high calculation cost in the finite element analysis of the Bailey beam solid model, a bar system model for the assembled Bailey beam is proposed based on the verified Bailey beam solid finite element model using the finite element analysis software ANSYS, and the modeling details of the solid model and bar system model are briefly described. It is also compared with the Bailey beam test in the Multi-purpose Manual for Prefabricated Highway Steel Bridges to verify the accuracy of the Bailey beam solid model and the member system model in the linear state. This paper carries out nonlinear buckling analysis, compares the differences between the two models in nonlinear state, and obtains the correct finite element model of the bar system through modification, and uses the modified bar system model to replace the solid model to study the ultimate bearing capacity under different conditions. The influence of initial geometric defects on the mechanical properties of Bailey beams is explored, and the selection reference of geometric defects for checking the ultimate bearing capacity of Bailey beams is proposed. The results show that the displacement value of the solid model under the same load in the linear stage is slightly larger than that of the rod model, and the maximum displacement difference in the midspan is 4%, but the displacement values of the rod model and the solid model at each measuring point are close to the test results, and the error range is less than 2%. The internal forces of the bar model and the solid model are basically consistent, and the maximum error of the average equivalent stress of the section is 2.78%; in the nonlinear stage, the instability modes obtained by the two models under different lateral bracing spacing are highly consistent, and the difference of ultimate bearing capacity is within 2.4%. The load-displacement curve is basically consistent, the failure mode is basically consistent, and the load-displacement curve of the member system model is in good agreement with the test curve. It is proved that the member model can replace the solid model for the ultimate bearing capacity analysis. The lateral support spacing of Bailey beams plays a decisive role in the failure mode of the structure. When the lateral spacing of Bailey beams is greater than or equal to 7.5 m, the overall buckling of the structure occurs. When the lateral support spacing is less than 5 m, the Bailey beam will be local buckling. The buckling mode is that the top chord of the mid-span element is subjected to out-of-plane bending failure, and obvious local buckling deformation occurs, and the Mises stress of the largest part of bending deformation reaches yield. When the Bailey beam is globally unstable, the overall geometric defect has a great impact on the bearing capacity of the structure, and when the structure is locally unstable, it is necessary to pay special attention to the local geometric defect of the structure. The stable bearing capacity is sensitive to the amplitude of the overall defect, while the in-plane local defect has little impact on the stable bearing capacity of the bailey beam, and the out-of-plane local geometric defect has a significant impact on the stable bearing capacity of the bailey beam. When considering the initial geometric defect with the out-of-plane initial bending amplitude of 1/1 000, the out-of-plane local defect of the chord, vertical bar and diagonal bar reduces the stable bearing capacity by 6.1%, 1.3% and 1.2% respectively, while the in-plane defect only reduces it by 1.0% 1.0% and 0.5%. Therefore, special attention should be paid to the out-of-plane deformation of the chord when selecting the bailey sheet.