Abstract: The cracking reasons of a ZG15Cr1Mo1V-B2 steel high preesure outer cylinder which cracked when pumping were analyzed by means of residual stress test, mechanical properties test, fracture analysis and metallographic examination. The results show that the cracking started from the repairing welding area where loose defects concentrated and the ductility of the material didn’t meet with the requirement. So the bearing capacity of the defect area decreased and the stress concentration was aggravated. At the same time, it has a high residual stress and a large hardness gradient at the repairing welding area. So it could be concluded that the main reasons for the cylinder cracking were that the casting defects were not cleaned thoroughly during repairing welding, the welding quality was not high enough and the stress elimination was inadequate.