Shrinkage cavities and large inclusions are serious internal defects of heavy steel ingot and influence the quality of subsequent forgings. In order to remove these two types of defects, a 70 t 12Cr2Mo1 heavy ingot fabricated by vacuum carbon de-oxidation process was sectioned and investigated by means of structure observation and EDS analysis. To further study the forming mechanism of shrinkage and inclusion defects and find possible solutions, simulation on pouring and solidification processes was also carried out using Fluent and ProCAST software, respectively. Results show that the shrinkage defects do not appear in the middle-upper part of the ingot. The critical value of shrinkage cavity criterion is ascertained as 0.013 on the basis of sectioning investigation and simulation results, which can be used in computer simulation to predict and avoid shrinkage defects in production of 12Cr2Mo1 ingots with different weights. However, large inclusions are found at the bottom of the ingot body. The bad thermal conditions of the ingot surface and large amount of entrained slag are the main origin of the large inclusions. The simulation result of the pouring process shows that large inclusions may be eliminated by combined measures of improving the top thermal condition and controlling the height of rudimental molten steel in the ladle to above 300 mm. Shrinkage cavities and large inclusions are serious internal defects of heavy steel ingot and influence the quality of subsequent forgings. In order to remove these two types of defects, a 70 t 12Cr2Mo1 heavy ingot fabricated by vacuum carbon de-oxidation process was sectioned and investigated by means of structure observation and EDS analysis. To further study the forming mechanism of shrinkage and inclusion defects and find possible solutions, simulation on pouring and solidification processes also also carried using Fluent and ProCAST software, respectively. Results show that the shrinkage defects do not appear in the middle-upper part of the ingot. The critical value of shrinkage cavity criterion is ascertained as 0.013 on the basis of sectioning investigation and simulation results, which can be used in computer simulation to predict and avoid shrinkage defects in production of 12Cr2Mo1 ingots with different weights. However, large inclusions are found at the bottom of the ingo t body. The bad thermal conditions of the ingot surface and large amount of entrained slag are the main origin of the large inclusions. The simulation result of the pouring process shows that large inclusions may be eliminated by combined measures of improving the top thermal condition and controlling the height of rudimental molten steel in the ladle to above 300 mm.