A steady magnetic field is applied to suppress the surface undulation after laser remelting without changing the laser processing parameters. A two dimensional (2D) transient multi- physics numerical model, concerning heat transfer, fluid dynamics, phase transition, electromagnetic field and the surface morphology of molten pool, is established. Comparing with the experimental results and numerical results, the suppressing mechanism of surface undulation related to the temperature field and velocity field in the molten pool is discussed. The results indicate that the Lorenz force supported by the steady magnetic field is a sort of drag force, whose direction is opposite to the melt flow all the time. With increasing the magnetic flux density, the velocity of molten pool is reduced gradually, while the temperature field remains changeless. When the magnetic flux density is higher than 0.5 T, the shape of molten pool is changed and the height of surface undulation is decreased obviously without changing the microstructure.