Abstract: The elongation after fracture of Q235B steel medium thick plate tensile specimen was unqualified in the mechanical property test. The reasons for unqualified elongation after fracture of the sample were analyzed by macroscopic observation, chemical composition analysis, metallographic examination, scanning electron microscope and energy spectrum analysis. The results show that the main reason for the unqualified elongation after fracture of the sample was that during the steelmaking process, the tundish temperature was high, and the speed of the continuous casting slab was slow, resulting in the segregation of S element. The ferrite segregation band was formed by MnS inclusion as nucleation point during rolling cooling. In the tensile test, the stress concentration was generated at the non-metallic inclusion of the sample, resulting in the delamination fracture of the steel plate, which reduced the elongation after fracture.