Abstract: The phenomenon of unqualified elongation after fracture of Q235B steel plates constantly appeared in a steel plant at the initial stage, the causes of unqualified elongation after fracture were analyzed by means of chemical composition analysis, mechanical property test, fracture surface analysis, metallographic examination and so on. The results show that the residual hydrogen elements in the steel plates, serious banded structure produced after rolling, center segregation and non-metallic inclusions resulted in the uneven composition of the steel plates and damaged the continuity of the metal matrix, which eventually led to the unqualified elongation after fracture of the Q235B steel plates.