The GB/T 24520-2009 standard specifies a method for the determination of lanthanum, cerium and mg in cast iron and low alloy steels by Inductance Coupled Plasma Atomic Emission Spectroscopy (AES). The method is suitable for the determination of lanthanum content in the range of 0.002% to 0.10%, cerium content in the range of 0.005% to 0.15% and mg content in the range of 0.003% to 0.15%. The samples are first analyzed with muriatic acid, nitric acid and perchloric acid, and then in the dilute test solution, the Inductance Coupled Plasma Atomic Emission Spectrometer is used to analyze the emission light spectrum intensity of the elements at the recommended wavelength measurement to determine the content of lanthanum, cerium and mg in the sample. This method ensures accurate analysis of these three elements in cast iron and low alloy steel.