A vast amount of information data must be input before finite element analysis of steel frames, such as node information, element information and load information. It is necessary to develop a computer aided design system with 3D interactive function. A CAD system for steel frame based on object ARX is developed with AutoCAD API and the Object -oriented technique. In this paper, some key programming technologies is studied, including design of structures, identification of floor system, also, it has presented the relevant implementing code.