Abstract: Coal mill pull rod of 3 044 t/h ultra supercritical boiler in a power plant fractured. The causes of fracture on pull rod were analyzed by means of chemical composition analysis, fracture analysis, mechanical property test and metallographic examination. The results show that the quality of heat treatment of pull rod was poor, which resulted in the unqualified indexes of strength and impact toughness, and the cold crack produced at the worn repair welding due to insufficient heat treatment. Under the action of long-term reciprocating tensile stress and bending stress, the fatigue crack propagated and finally led to fatigue fracture of pull rod.