A parametric modeling method, one of the core technology of BIM (Building Information Modeling), is proposed for efficient 3D assembly design among components of a superstructure module of modular steel bridge. Assembly system is classified into 3 levels as LoD (Level of Details) for 3D assembly design of the parts. Components forming 3D shape of the parts are identified and defined as parameters, variables depending on parameters, or constants independent of the parameters. Then, spatial assembly rules among the parts are defined according to the assembly system. Positional relations among the identified shape components are defined for mating spatial position and geometrical relations are defined for constraining degree of freedom on X, Y, and Z axis. Finally, a standardized template is designed by applying the rules to 3D based assembly design for the parts of the superstructure module. In addition, applicability of the parametric modeling method is demonstrated by testing the shape variation of the superstructure module according to changing the defined parameters.