Abstract: For the diffecrent degress surface crack occur in the prodution of steel 12CrlMoV,it was confirmed by metallographic microscope and scanning electron microscope that cracks in the rolled material were caused by cracks in the cast billet. Based on the analysis of the causes of the cracks ,it is concluded that the cleanliness of molten steel, the nitrogen content of molten steel and the steel shielding slag in mould are the main reasons for the cracks. Improve the cleanliness of molten steel by controlling the terminal carbon of the electric furnace, optimizing the refining slag system and treating the deoxygenation products with calcification, controlling the nitride precipitation on the surface of the billet, ensuring the N content of steel < 0.007% ,and using pipe billet steel shielding slag, the surface crack defect rate of 12CrlMoV steel decreased from 1.6% to 0.3%.