Abstract: Defects appeared on the surface of a 20 round steel. The causes of longitudinal crack defects on the surface of the steel were systematically analyzed by means of macroscopic observation, oxygen, nitrogen and hydrogen analysis, chemical composition analysis, metallographic examination, scanning electron microscopy and energy spectrum analysis. The results show that the reason for the surface defects of the material was that the longitudinally distributed cracks were formed on the surface of the round steel after the surface and near-surface bubble defects of the continuous casting billet were heated and rolled. Finally, the improvement measures were put forward, and the results could provide strong technical support for improving the production process.