Tensor field design plays an essential role in various computer graphics applications. One of the main challenges in field design is how to obtain a smooth field preserving meaningful singularities presented in the original scene. Compared with well-studied point singularities, line singularities, commonly occur on object boundaries and occluding contours, are not exploited enough for field design in previous work. In this paper, we discuss the definition of line singularities, and introduce a line-singularity-based interactive tensor field design method, allowing the user to design feature-preserving tensor fields with less effort and to preserve both input singularities and user-specified stroke directions. To avoid introducing extra interaction burdens to the user, our method automatically locates line singularities using a geodesic-based segmentation. We demonstrate the capabilities of our method on tensor field design with various nonphotorealistic rendering applications and the real-Time performance accelerated on GPU.