φ200 mm silicon single crystals were grown in the φ450 mm hot zone of a Czochralski (CZ) face. By modifying the patt and the velocity of the argon flow, the silicon single crystals with different oxygen concentrations were obtained. Through numerical simulation, the velocity of the argon gas flow was plotted for the first time. The experiment results were analyzed and the optimum condition of the argon flow with the lowest oxygen concentration was obtained.