In this paper, the method of quantitative analysis by GDMS of Fe, Si, Cu and other nine impurity elements in pure magnesium was studied. The applicable method for the preparation of samples were confirmed. The isotopes, resolution and the analysis conditions were optimized. In addition, RSF of these impurity elements was obtained using standard samples in order to corrected the standard RSF. The results showed that using wire cutting introduces less impurities. When the discharge current was 55mA and the gas flow was 350mL/min, matrix signal was stable and suitable. Quantitative analysis of samples with the RSF314 showed a better precision and accuracy than using the standard RSF. Comparison with the results of ICP-AES, AAS, ICP-MS, the testing results were closer to the standard value which were obtained by our studied. But our method was more convenient on sample preparation, faster analysis speed, and higher overall accuracy than other means.