By analyzing the system of argon stirring molten steel in ladle, many affecting factors are determined. The model of characteristic number about describing mixing effect of steel stirred by argon in the ladle is gotten by dimensional analysis. Taking a ladle furnace of some steel plant as prototype, hydraulics simulation experiment is designed by generalized similarity principle. The equation for characteristic number about the mixing effect of steel in the ladle is gotten by stepwise regression, which is suitable for ladles whose geometric structure are similar to the ladle in this study. The results could be gotten as follows. The four decisive characteristic numbers that are considered are all significantly effective to H0. And gas-liquid density ratio πρ is the most remarkable among them. When doing some hydraulics experiment to simulate the phenomenon of molten steel stirred by argon from the bottom of ladle, using some conventional gas instead of argon could result in a great difference between the decided characteristic numbers of model and prototype.