Abstract: As the “heart” of continuous caster, the flow field of mold directly affects the quality of the slab. For a billet caster, in-mold electromagnetic stirring (M-EMS), as its necessary configuration, can improve the flow field in the mold, homogenize the liquid steel temperature, improve segregation, and improve the slab quality. This paper utilized a 410 mm × 530 mm large billet caster in a factory, which is one of the largest section casters in China. Based on it, a three-dimensional numerical model was established using the ANSYS finite element software to study the influence of electromagnetic stirring on the flow field, liquid level fluctuation, and temperature field of the mold. After electromagnetic stirring was applied, the liquid steel was subjected to a radial electromagnetic force, and the liquid surface shows a rotating flow trend. The maximum tangential velocity of molten steel increases with the increase of current and decreases with the increase of frequency. When the current of electromagnetic stirring increases from 0 A to 500 A, the fluctuation of the liquid level increases from 1.21 mm to 4.35 mm. The maximum tangential velocity of the electromagnetic stirring center increases from 0.02 m∙s⁻¹ to 0.21 m∙s⁻¹. Electromagnetic stirring can restrain the impact of the high-temperature jet from the nozzle, move the high-temperature zone of molten steel upward, and make the temperature of molten steel more uniform. Under the action of a radial electromagnetic force, the horizontal swirl of liquid steel can inhibit the growth of the primary shell, reduce the growth rate of the shell, and reduce the thickness of the shell out of the mold by about 2.3 mm. The comprehensive analysis shows that the reasonable current of electromagnetic stirring is 400 A and the frequency is 1.5 Hz. At this time, the fluctuation of the slag level is about 2.73 mm, and the temperature field is relatively uniform.