Abstract: The ultra-high strength seismic steel bar broke during the bending test. The reasons for the fracture of the bending sample were analyzed by means of macroscopic observation, chemical composition analysis, scanning electron microscope analysis, metallographic examination and central segregation inspection. The results show that the heating temperature before hot rolling was higher, and the austenite grain grew after deformation, which increased the size of pearlite colony and the content of pearlite. The slow cooling resulted in the precipitation of network ferrite along the austenite grain boundary, which reduced the plasticity and toughness of the material, and finally led to the fracture of the sample during the bending test.