A model for conceptual design of mechanical devices is studied based on qualitative simulation. In this model, the desired functions are expressed by state-transit-diagrams(ST-diagrams) and design space is represented by qualitative-state-curves(QS-curves). The first design idea, called seeds idea, is proposed by the designer and then is abstracted into QS-curves. The qualitative simulation is implemented based on the QS-curves. By changing the motion of acting parts, the connection of parts and the motion of driving part, new design ideas are generated. With this model, a series of new design ideas derived from the seeds idea can be achieved. A model for conceptual design of mechanical devices is based on qualitative simulation. In this model, the desired functions are expressed by state-transit-diagrams (ST-diagrams) and design space is represented by qualitative-state-curves The first design idea, called seeds idea, is proposed by the designer and then is abstracted into QS-curves. The qualitative simulation is implemented based on the QS-curves. By changing the motion of acting parts, the connection of parts and the motion of driving part, new design ideas are. With this model, a series of new design ideas derived from the seeds ideas can be achieved.