In this study, a modified Curie–Weiss model was established for the magnetic susceptibility of high-purity molybdenum and Mo–La alloy powders. The elemental composition was analyzed by GDMS, and combined with the M–T and M–H data measured by SQUID, the temperature-independent contributions of weakly magnetic elements such as La and the paramagnetic contributions of impurity ions such as Fe, Co, and Ni were distinguished. Based on the parameters obtained from the nonlinear least squares fitting, the deviation between the magnetic susceptibility at room temperature calculated by the model and the experimental value was within 5%. The results show that this model can reasonably describe the influence of trace impurities on the magnetic susceptibility of the system and provides an effective method for the magnetic prediction of high-purity metal powders.