Abstract: The spherical tank is one kind of very important pressure vessels,and it is widely applied in the petrochemical industry. The scientific methods were used to find the cause of cracks for liquid ammonia spherical tanks of Q370 R steel. The correct welding process was not strictly performed,which results in the high hardness and large residual stress of the welded joints,and leads to the hydrogen-induced cracks.Under the action of the medium,a large number of stress corrosion cracks are initiated. The criteria of repairing by the hardness standard and the points of welding operation were determined,which can not only guarantee the quality of the repairing work,but also prevent the occurrence of new cracks.