The invention discloses a method for improving internal quality of a casting blank. In a refining process, a silicon-calcium wire is added before soft argon blowing of LF (Ladle Furnace) refining, argon gas is required to be closed during addition of the silicon-calcium wire, soft argon stirring is adopted after addition, the soft argon blowing time is required to be controlled to be over 16 minutes, and the amount of added silicon-calcium wire is 1.5-3.0m per ton of steel; and in a casting process, a rare earth wire is added into a crystallizer, the amount of added Re wire is different according to different continuous casting speeds and S contents in molten steel: 22-24m of rare earth wire is added per minute when the continuous casting speed is 0.7-0.725m/min and S content in the molten steel is more than or equal to 0.015 percent; and 8-9m of rare earth wire is added per minute when the continuous casting speed is 0.7-0.725m/min and S content in the molten steel is less than 0.015 percent. The molten steel in the same melting process is treated through above-mentioned methods in sequence, so that the improvement of purity of the casting blank is promoted, the segregation degree of the casting blank is greatly relieved, and thereby, the internal quality of the casting blank is effectively improved.