Abstract: Conceptual design is an important phase in product design, its main task is to work out spatial layout and identify the mating relationship. Because of uncertainty of product structure in the conceptual design stage, many of the 3D CAD tools can hardly support such kinds of activities for conceptual layout. The author abstracts the assembly relation into 3D geometric constraints and uses these constraints to drive the conceptual design activities, thus realizing a top-down design mode in the accordingly developed 3D CAD tool.