The electric resistance of Sn-20%Pb alloy was measured at different temperatures when the AC magnetic field was applied. The results showed that the AC magnetic field had no effect on the electric resistance of solid Sn-20%Pb alloy, but when it was applied on the liquid Sn-20%Pb alloy, the electric resistance of alloy increased gradually. After the AC magnetic field was stopped, the electric resistance remained at a constant value for a long time, and then slowly decreased to the value before applying the AC magnetic field. The results of metallographic microscope and SEM showed that the AC magnetic field not only refined the primary β-Sn grain, but also increased the Pb content in primary β-Sn grain, so that the distribution of Pb was more uniform than that in the sample without magnetic field. The Anderson's theory of disorder has been used to explain the experimental results. It was considered that the electromagnetic stirring caused by AC magnetic field destroyed the short range order of the Sn-20%Pb melt, improved the disorder degree of the system, transformed some extended states electrons into localized states electrons,consequently increased the electric resistance of the melts.