The refinement mechanism of primary dendrite spacing caused by a transverse static magnetic field during directional solidification was proposed and verified experimentally in this paper. According to the Kurz-Fisher model, this magnetic field-induced refinement is related to the temperature gradient and solute redistribution in the mushy zone during directional solidification. Therefore, the effect of the magnetic field on the temperature gradient was investigated and the solute redistribution induced by the thermoelectric magnetic convection was measured by Rank sort method. The experimental results indicate that the refinement of primary dendrite spacing can be attributed to the solute redistribution rather than the change of temperature gradient.