Abstract: An exhaust branch pipe was integrally casted with QTRSi4Mo heat resistant cast iron, and cracking occurred after about 2 000 h of operation. The cracking causes of the exhaust branch pipe were analyzed by direct reading spectrometer, tensile testing machine, scanning electron microscope, optical microscope and other equipments. The results show that the cracking of the exhaust branch pipe was mainly due to the existence of much shrinkage defect inside the material, which reduced the strength of the material. Under the vibration force during service, the exhaust branch pipe cracked at the defect.