A rotating stopper-rod technique was proposed to suppress the formation of free-surface vortex in the tundish. The large eddy simulation model coupled with volume of fluid model was developed to study the steel–slag–gas three-phase flow behavior. The critical slag entrapment height of the free-surface vortex and mass of residual steel were predicted at different rotating speeds (30, 60, 90 and 120 r/min) of the rotating stopper-rod. The numerical model was verified by water model experiment. The results showed that by rotating the stopper-rod in the opposite direction of the vortex above the submerged entry nozzle, the formation of vortex can be effectively disturbed and the critical height of the free-surface vortex can be reduced. Particularly for the 2nd strand, when the rotating speeds are 30, 60, 90 and 120 r/min, the critical height of the free-surface vortex above the 2nd strand is 7.3, 4.7, 6.3 and 7.4 cm, respectively. A reasonable rotating speed should be 60 r/min, which can reduce about 2 tons of residual steel. Other rotating speeds just can reduce about 1.6 tons of residual steel.