Through the comparison with main contents among JTJ025—86 Standard for Design of Steel Structure and Timber Structure Highway Bridges and Culverts , TB10002.2—2005 Code for Design on Steel Structures of Railway Bridge and GB50017—2003 Code for Design of Steel Structures, the similarities and differences of design criteria, the steel grades and material properties, design load, the main calculation formula and so on between highway, railway steel bridge codes and building steel structure specifications are researched, then the reasons for the differences in the material strength values, structural strength, stability and fatigue checking between the three kinds of specifications are analyzed particularly, and the rationality of main checking formula in highway, railway steel bridge specifications is evaluated objectively. For the project planners, the research is of practical value to comprehend the design code of steel structure in rail road bridge and grasp the essence of steel structure design.