The paper focuses on the pressure information application of digital pen in sketching system and proposes a novel design pattern based on the direct conversion from the pressure information to the threedimensional stoke depth. The method firstly defines the curve modification region and the pressure scale, determines the stroke influence area of the plana or spatial curve with the simplified method of normal pressure estimation on each node of stroke point. Then the pressuresensitive stoke was smoothly shown with the pressure information by width and color. Finally, the pressuresensitive information was converted to the depth information of a planar stroket in order to create a threedimensional stroke with the endpoint locked and the parabolic pressure conversion methods. The conversion method based on the pressuresensitive digital pen was applied in the new version of automotive threedimensional freehand sketching system. The experimental results show that the pressure information of digital pen can be effectively converted into the depth information of a planar stroke, which can be changed into a spatial curve directly. The calculation time of the conversion was short, and the visualization effect was satisfied obviously. In addition to the application during the concept design phase of the automotive 3D sketching modeling, this conversion can also be applied to mechanical design and other engineering design areas.