The invention relates to a method for purifying impurities in metal under a transverse magnetic field. The Seebeck effect exists in the directional solidifying process, if the transverse magnetic field is exerted, thermoelectricity magnetic force and thermoelectricity magnetic flowing are generated through interaction of the magnetic field and current, and the impurities are segregated on one side of a crucible through the liquid-phase flowing function via force or flowing. A directional solidification microstructure with the impurity space distribution change is obtained. The technology method is characterized in that the whole directional solidification process is finished under the 0-1T (Tesla) of the transverse static magnetic field, a fine and long alloy bar is adopted as a sample, the downward drawing and pulling rate of the sample is 1-100 microns/s in the directional solidification process, after the stabilizing state is reached in growth, the sample is rapidly pulled into a cooling medium to be quenched, and finally the directional solidification microstructure with the impurity space distribution change is obtained. The impurities are removed through a physical cutting method.