<p>Composite components are usually embedded trimming baselines for guiding the edge trimming. The baseline is extremely thin and long, which poses challenges on the automatic measurement of its position and normal. This paper proposes a progressive measurement method for extremely thin and long trimming baselines on complex surfaces. Line laser scanning scheme is used for achieving automatic and on-machine measurement. The proposed extraction method of the baseline from the measurement point cloud involves the accurate locating of any starting point and full traversing of the whole baseline. The starting point is searched from several locating regions along the baseline direction and accurately extracted from the locating region through fine geometric analysis. Based on the continuity of the trimming baseline, all representative points are obtained by progressive traversal alone the baseline starting from the starting point, where the intermittent positions are determined by prediction. To satisfy the toolpath smooth requirements, the representative points of the trimming baseline are further smoothed through B-spline fitting. The normal of the representative points of the trimming baseline are calculated from the corresponding local region, and are also smoothed. Robustness and accuracy experiments are performed to verify the effectiveness of the proposed method. The results show that proposed method has high robustness facing uneven workpiece surface, and intermittent trimming line and curved surface, achieving the measurement error within 0.094&#xa0;mm. This method enables precise measurement of extremely thin and long baselines on composites with complex surfaces.</p>