The invention relates to a preparing method of starch syrup for carbon black. The method includes the steps of starch liquefacation, primary saccharifying, combined saccharifying, filtering, refining and concentrating, maltose transglycosylase is added into complex carbohydrate, heat is preserved at the temperature of 40-55 DEG C, and saccharifying is carried out for 10-20 h. Combined saccharifying is carried out after primary saccharifying, the ash content of the carbon black is reduced, the carbon black particles are moderate in hardness, and the quality of carbon black products is improved.