Abstract: Regulatory effect of trace La and Sc on grain boundary segregation of impurities in Al-Mg as-cast welding materials were studied using optical microscopy, scanning electron microscopy, energy spectrum analysis and X-ray diffraction. The results show that there was Al-Mg impurities segregation on the grain boundaries of welding material after solidification. The addition of trace La and Sc could reduce the gathering of Fe, Si impurities around the grain boundary. The effects of La and Sc on the grain boundary segregation vary significantly. The decrement of grain boundaries segregation degree of the Sc introduced alloy is superior to La introduced alloy. The reason of this discrepancy came from the different regulation mechanisms of the impurities' grain boundary segregation.