A third generation single crystal superalloy (WZ30) was used to pour single crystal test bar in the returned alloy ingot. There were serious stripe defects on the surface of the castings, which originated from filamentous or flocculent oxide inclusions. The sliver defects on the surface of the parent alloy ingot and the castings were analyzed by EDS spectrum anakysis. The results show that the streaks on the surface of the casting are caused by alumina inclusions in the parent alloy. Oxidation inclusions become liquid scum during melting, and enter the mold cavity with metal liquid in flocculent form during pouring, and attach to the inner wall of the mold shell. In the process of directional solidification, these inclusions embedde in the dendritic stem, which greatly reduce the strength of the dendritic stem. Under the action of various stresses, these inclusions fracture and deflect into sliver grains.