Abstract: The incomplete opening of the slide plate is prone to inducing biased flow in the mold and non-uniform growth of the solidified shell, which critically affects slab quality. Electromagnetic stirring (EMS) is an effective method for regulating the mold flow field, yet further research is needed to quantitatively evaluate its improvement effect on asymmetric flow and solidification behavior under slide-gate control conditions. This study aims to reveal the mechanism of biased flow and investigate the regulation laws of EMS on the asymmetric flow field and solidification behavior. Focusing on a slab mold in a steel plant, a three-dimensional large eddy simulation (LES) model coupling electromagnetic, flow, and thermal fields was established. A "symmetry index" was introduced as a quantitative indicator to systematically investigate the effects of stirring current and slide gate opening rate on the flow field structure and shell uniformity. The results indicate that the asymmetry of the flow field inside the submerged entry nozzle (SEN) is the root cause of the biased flow in the mold. Without EMS, the jet deflects toward the inner radius (IR) side, leading to severe remelting of the IR shell. The application of EMS generates a horizontal recirculating flow that effectively reduces the velocity difference between the two sides of the nozzle, significantly improving flow field symmetry. As the stirring current increases, the average symmetry index rises from 0.62 to 0.71, and the remelting of the IR shell is alleviated; however, excessive current intensifies the scouring of the narrow face shell. The standard deviation of shell thickness decreases first and then increases with rising current, reaching a minimum of 0.215 mm at 600 A where shell growth is most uniform. Furthermore, increasing the slide gate opening rate helps enhance flow field symmetry; as the linear opening rate increases from 50% to 100%, the average symmetry index rises from 0.64 to 0.75, and the standard deviation of shell uniformity decreases to 0.202 mm. In conclusion, EMS can effectively mitigate asymmetric flow caused by slide-gate control, but the stirring intensity must be optimized to balance flow symmetry and shell scouring. Considering both flow field symmetry and shell quality, the optimal stirring current is determined to be 600 A under a 60% slide gate opening condition. These findings provide theoretical guidance for optimizing EMS process parameters and improving slab quality in industrial slab continuous casting applications.