According to the principle of topological projection invariance, the spatial graph contour extraction problem is processed in twodimensional projection plane. In order to automatically determine the projection plane, a method of normal vector dynamic composition is put forward. In addition, deformation parameter is introduced to validate the effectiveness of spatial closed contour, and the corresponding calculation method is given. The mathematical model of spatial graph contour extraction is established and the extraction method based on the model is constructed. The practical application verifies the stability and practicality effectiveness of the method.