Penetrating cracks appeared in the centre of the heat-resistant steel 10Cr10NiMoW2VNbN blank of ultra supercritical unit blade bolt after forging and quenching. Hardness tests show the lower values and optical metallographic observation shows a lot of similar inclusions at the cracks. By SEM and EDS analysis it is found that the components of the cracks are complex and significantly different from that of the matrix. The results show that the foreign smelting slag causes excessive non-metallic inclusions in the forging center and induces the cracks propagating in the forging process. To avoid such raw material defects from happening again, it is recommended to improve the level of smelting and take strict inspection procedures of factory products, while increasing the ultrasonic testing procedures.