Abstract: In order to improve the solidification structure of steel 20CrMnTi billet, based on the CAFE model in ProCAST software, the solidification structure is numerically simulated, and the effects of different superheat of molten steel, casting speed of billet and secondary cooling water ratio on the solidification structure are studied. The simulation results show that with decreasing the superheat of molten steel, and increasing cast speed, and decreasing the specific water a- mount of secondary cooling can increase the equiaxed grain ratio and refine the grain size of the billet, and the superheat of molten steel has the most influence on it； the equiaxed grain rate increases by 3. 7% with the decrease of superheat by 10°C ；the equiaxed grain ratio of the casting billet increases by 1. 8% on average when the cast speed increases by 0.1 m/ min； and the equiaxed grain ratio of the billet increases by 1. 65% when the specific water amount decreases by 0. 1 L/kg. Production application shows that with superheat of molten steel 30 °C，as cast speed decreases from original 2. 2 m/min to 2. 1 m/min, secondary cooling specific water amount inceases from 0. 6 L/kg to 0. 7 L/kg, the billet center porosity obvously decreases.