To mitigate the problem of distance measurement errors in the existing vehicle ranging method based on lane line width of curved roads, this study propose a lane line lateral width estimation method at the curve based on the slope of the lane line. Further, an improved ranging model based on lateral width of the lane line is derived. Then, compared with the existing method, the ranging accuracy of the proposed method is obviously improved under the concentric circular lane line model. When the curvature of the lane line is 0.01 and 0.005, the distance measurement errors of proposed method within the true distance of 50 and 100 m are less than 3% and 1%, respectively. Finally, the proposed method is evaluated based on an actual road environment on the KITTI dataset. Results show that the average distance measurement error is less than 5%, indicating the accuracy of distance measurement is significantly improved at the curved road.