<p>A three-dimensional mathematical model, combining the large eddy simulation (LES) turbulence model and the volume of fluid (VOF) multiphase model, was developed to quantitatively compare the effect of the casting speed, submerged entry nozzle (SEN) submergence depth, and mold width on the multiphase flow, level fluctuation, slag entrainment, and steel jet oscillation during the slab continuous casting process with a straight through nozzle. The mass of the slag entrainment increased periodically in a stepped manner over time. The slag entrainment rate was 2.86 × 10⁻⁴ kg/s with a 1.2 m/min casting speed, 110 mm SEN submergence depth, and 735 mm mold width. The slag entrainment rate increased with the greater casting speed and mold width, while the SEN submergence depth had little effect on it. The jet oscillation frequency and the variation frequency of the slag entrainment were determined through the Fast Fourier Transform (FFT) analysis. The slag entrainment frequency was approximately twice that of the jet oscillation frequency. The casting speed and mold width significantly impacted both the jet oscillation frequency and slag entrainment frequency, whereas the SEN submergence depth had little effect. An empirical expression for the jet oscillation frequency (<i>f</i>_O, Hz) relative to casting speed (<i>V</i>_c, m/s) and mold width (<i>w</i>, m) was fitted as <i>f</i>_O=0.0513+2.506<i>V</i>_c−0.0705<i>w</i>.</p>