Abstract: In order to analyze the causes of surface cracks of 20 round steel, the metallographic microstructure,micro-morphology, crack composition and low-power morphology of the cracks were analyzed by means of metallographic microscope, SEM, EDS and acid pickling inspection. The results show that the surface crack of the 20 round steel is caused by the presence of subcutaneous bubble defect in the casting billet, which is exposed after the heating before rolling, and the subsequent cracking and expansion in the rolling process.