Based on the similarity principles and the operating condition for a slab continuous casting，a water modeling system for mold with the size ratio 1:2.5 to 1 500 mm × 280 mm slab mold was established. The influence of the surface fluctuations with casting speed, nozzle angle, argon flow and immersion depth were studied, as well as the slag entrapments were analyzed at the experiments. The results show that powder entrapment by bubble groups attack and powder entrapment similar to vortex are the main ways with the argon blowing.