Based on multi-physical field coupling numerical simulation method, magnetic field distribution, melt flow, and heat transfer behavior of a Φ300 mm AZ80 alloy billet during differential phase electromagnetic DC casting (DP-EMC) with different electromagnetic parameters were studied. The results demonstrate that the increase in current intensity only changes the magnitude but does not change the Lorentz force's distribution characteristics. The maximum value of the Lorentz force increases linearly followed by an increase in current intensity. As the frequency increases, the Lorentz force's r component remains constant, and the z component decreases slightly. The change in current intensity correlates with the melt oscillation and convection intensity positively, as well as the liquid sump temperature uniformity. It does not mean that the higher the electric current, the better the metallurgical quality of the billet. A lower frequency is beneficial to generate a more significant melt flow and velocity fluctuation, which is helpful to create a more uniform temperature field. Appropriate DP-EMC parameters for a Φ300 mm AZ80 Mg alloy are 10-20 Hz frequency and 80-100 A current intensity.