Abstract: The cracking cause of hot supply pipeline was analyzed by means of macroscopic examination, chemical composition analysis, metallographic examination and so on. The results showed that lots of tertiary cementite precipitatea long the ferrite grain boundary due to the improper heat treatment. The reticular and line distribution of tertiary cementite resulted in the great reduce of the plasticity and ductility of the hot supply pipeline, and finally led to cracking on the hot supply pipeline. At last the measures were presented.