Research conclusions:Based on the concept of the interactive software design，an object － oriented model was established for design of the subgrade’s cross section to have the subgrade’s cross section made to order． According to the design process of the cross section based on message － driven，the logic relationships among the various designed entities were concentratedly managed by their AcDbObjectId pointer in the cross section model． In order to design the huge sections in similar environmental conditions，the piecewise parameterization method was adopted． The user could draft and edit the graphics by elastic － effects dragging because the graphic entity＇s pinch point functions were reloaded．The users could define the design drawing by theirselves because the entity＇s drawing functions were extended． This system could real － time response the user＇s design operation． The design parameters could be modified by editing graphics directly for strengthening the user experience． Moreover，the system could cope with the a large number of batching designs and particular customized design． Finally，the real － time interactive CAD was established for the subgrade’s cross section based on graphics － data interactive driven．