In this study, the authors proposed a novel method to extract 2D projection contour from 3D models. 2D projection contour is a significant feature and data of a 3D shape. It can be used in the fields of 3D model retrieval, analysis of shape, free distortion and joint of curved surface. Triangular mesh is generally adopted as the data representation form of 3D models. Therefore, as a case study, we consider 2D projection contour extraction from triangular mesh of 3D models. In their approach, the authors performed a discriminant of the visualization to every 3D model curved surface and get the boundary vertexes of them, extract contour vertexes after projection, construct rings and perform a ring-relationship-based judge mechanism to get the 2D projection contour. Their method is unique in that we can extract contours from both single-valued and arbitrary complex space curved surface by ring-relationship-based strategy. Theoretical analysis and experimental results show that they can significantly improve the accuracy and efficiency in contour extraction by using ring-relationship-based method.