The electromagnetic dam (EMD) in twin-roll strip caster was simulated with FEM. The results show that the direction and magnitude of Lorentz force distributing in the EMD changed in a period. The negative part is about 30% of the positive part, and the acting time of the negative force is about 40% of a period. In order to prevent the liquid steel from leaking efficiently, the distance between two poles should be the same from the top to the bottom of the EMD, otherwise the magnetic induction will be very small at the top and large at the bottom.