Abstract: The mixing behavior of molten steel and slag has an important impact on refining effect in vacuum degassing (VD) process.In the VD refining process, the vacuum condition makes the bottom argon gas agitate the molten steel vigorously, and increase the exposed molten steel area, which not only aggravates the degree of steel slag mixing in the ladle, but also promotes the degassing reaction in molten steel.Moreover, it has an important influence on the desulfurization effect of molten steel.In the actual melting process, the exposed situation of the molten steel can be observed over the slag, but the steel and slag mixing situation in the ladle cannot be observed.Therefore, it has a great practical significance to study the slag mixing behavior in VD refining process under the different process conditions.In the water model experiment, the critical condition of half-mixing and complete-mixing was first defined.And then the impacts of the total blowing flow rate, vacuum degree, and the bottom blowing positions on half-mixing and complete-mixing were discussed.Besides, the effects of critical stirring energy density and stability of flow field on mixing behavior were also analyzed.The results indicate that half-mixing and complete-mixing critical gas flow rate decreases with the increase of vacuum degree in the laboratory condition.And the critical gas flow rate in 2/3R+2/3R is greater than that in 2/3R+center hole.In addition, the fitting formulas of vacuum degree and critical gas flow rate were given.Based on fitting formulas, critical gas flow rate in actual VD process was obtained when vacuum degree was 67 Pa.According to the results, the critical stirring energy density is nearly a constant when the steel-slag mixing reaches the same condition in steady flow field, which indicates that flow character can further influence mixing effect and it is affected by the bottom blowing positions.