In order to realize the fast evaluation and the dynamic adjustment and control of nodularizationeffectiveness for ductile iron preparation in the front of furnace，a mixture of the chemical reagent FeS，Fe2O3 and Te of moderate amounts in use was added in the thermal analysis sample cup for a modified cooling curve of the testing melt. Then，the quick testing method of active Mg content in nodularized melt was studied，of which the prediction equation for Mg content calculation was obtained by regression analysis between the spectrum analysis results of Mg and the related solidification characteristic parameters on the cooling curve in this paper. The experimental results show that the testing method is reasonable. Within the scope of the Mg content of 0. 025% ～ 0. 06% ，the predicted value Mg% of magnesium content is correlated with corresponding thermal analysis solidification characteristic parameters significantly，so the calculated results are reliable. For the 120 statistical sample data，more than 80% of the prediction error is within ± 0. 004% for the regression formula of the Mg content，meeting the control requirements on line.