Abstract: ARX（AutoCAD Runtime eXtension） is a new developing environment for AutoCAD system．Its object-oriented feature has brought a profound change in the development model for CAD software．The paper analyzes and exemplifies the mechanism of the object-oriented feature of ARX as taking the architectural stairways as an example．Some other key technical problems are also discussed，such as management policies for object name，virtual UCS，graphical user interface （GUI） and public library．All the discussion are tested and completed in a CAAD system ——AHD （Architectural House Design） ．