According to the macroscopic distribution of black line defects at surface, and the optical characteristics of microscopic cross-section and surface, combined with the tracking and analysis on the process and equipment of rolling production line, the cause and formation mechanism of black line defects on surface edge of hot continuous rolling carbon steel were determined. The results show that the occurrence of black line defects on the surface edge is related to the plastic deformation of metal on the edge during rough rolling, and the poor surface quality of vertical roller will promote or aggravate the black line on the edge. Compared with chamfered billet, the edge of right-angle billet is easy to form black line defects because of the large thickening of its edge and the large temperature drop. After the improvement measures such as optimizing the rolling mode, shortening the service period of vertical roller and putting in chamfered billet, the black line occasionally appears at surface for hot continuous rolling carbon steel and is within 10 mm from the edge, the black line defects on the hot rolled edge can be effectively eliminated.