The interfacial behavior between fluid steel and molten slag layer in a slab continuous casting mold with blowing argon gas was described using the VOF (volume of fluid) method and Lagrange multi-phase flow model, and the prediction was validated by the water model. The influences of casting speed, argon gas volume flowrate, mold width, submergence depth of SEN (submerged entry nozzle) and bubble size on interfacial behavior were numerically investigated. The results show that given a casting speed 1.8 m/min, the upper circulating flow in the mold gradually disappears and the interfacial unevenness disturbed by argon bubbles is gradually evident with increasing argon gas flowrate. Raising the casting speed from 1.2 m/min to 2.2 m/min leads to the deeper penetration of bubbles and less influences of argon bubbles on the flow pattern and interfacial profile at a given argon gas flowrate. Increasing the submergence depth of SEN can effectively restrain interfacial oscillations in mold, while mold width has little effect on it. Bubble size has a remarkable influence on the interfacial behavior of molten steel and slag in mold.