As for the increasing demands of offshore oil and gas, the new platforms are designed in more and more serious conditions. The safety check and modification of the complex structure has become a hot subject. Normally, the deck girder can be regarded as the combination of truss works and girders, which will suffer from the axial compression from global bending as well as shear force due to the local equipment loads. The main goal of this paper is to make contributions to predict the ultimate strength and post-buckling behavior of this kind of hybrid deck girder. Comparison among various cases is the main methodology for drawing a conclusion. The linear eigenvalue analysis is aimed to introduce the initial imperfection into the structure. The effects of imperfection on the ultimate strength are found to be small for this kind of robust structure. The way how the stiffener strength and boundary conditions influence the ultimate capacity is also investigated. In addition, two nonlinear finite element software (Abaqus and USFOS) provide similar pre- and post-buckling behavior, while the deviation for the ultimate strength is in a quite large but acceptable level. The simplified estimate according to design standards is also investigated here and the comparison with finite element analysis shows that the regulations can give a quite good prediction about the ultimate capacity of stiffened plates and primary girder section.