9Cr3Mo cold roll steel ingot of Ф600 mm was produced in Xingtai Iron & Steel Corp., Ltd. by using a pulled-ingot type electroslag furnace with protective atmosphere, which was forged into a roll blank with the specification of Ф440 mm×1700 mm×4215 mm. Then, after forging, the continuous defects with the diameters of Ф4-Ф5 mm and the range length of about 330 mm were found in the center of roller diameter by ultrasonic flaw detection. Furthermore, the internal flaw detection of the cold roll steel was analyzed comprehensively by chemical composition analysis, metallographic examination, macro-microscopic examination of fracture and pickling examination. The results show that after pickling, there are dense longitudinal micro-cracks in the center of 9Cr3Mo cold roll steel, which looks like loose material defects. However, it is found that there is no decarbonized layer near the crack by further inspection, the surrounding structure is the uniform spheroidized annealed structure, and there are no obvious abnormalities in the inclusions and no segregation of chemical composition, which rules out the defect of loose structure for electroslag ingot. Then, the characteristics of the defect were analyzed, and it is determined that the defect is a tensile crack during the forging deformation due to insufficient penetration of the core and low material plasticity during the forging process. Finally, by optimizing the holding time between each fire during the forging and heating process and increasing the preheating of manipulator clamp, the forging temperature of the cold roll steel core is ensured, and the abnormal flaw detection of such forging rolls is avoided.