Abstract: Taking the round billet mold of Φ450 mm in a steel plant as the prototype, the water simulation experiment of mold was carried out to study the effects of drawing speed, nozzle immersion depth and blowing flow on the liquid level fluctuation, flow field and slag entrainment in the mold, and to determine the best process operation parameters of the mold. The results show that the increase of blowing flow and the decrease of nozzle insertion depth will aggravate the fluctuation of liquid level in the mold and increase the probability of slag entrapment. When the blowing flow rate and nozzle insertion depth are fixed and the pulling speed changes in the range of 0.40-0.46 m/min, its influence on the mold flow field is not obvious. Whether at the current drawing speed (0.40 m/min) or at the lifting speed (0.42, 0.44, 0.46 m/min), the best process operating parameters are that the nozzle insertion depth is not less than 140 mm and the blowing flow is not more than 1.12 L/min.