The technique of laser induced breakdown spectroscopy (LIBS) was applied to analyze the composition of liquid steel. The results of research indicated that oxidation of liquid steel on the surface could be avoided because the argon and the plasma signal was enhanced at the same time. The surface of liquid steel was excited by high-energy pulse laser and plasma was formed in argon atmosphere. The spectral signal was collected with a multi-channel CCD spectrometer when the plasma cooled off and relevant information about the composition of samples was obtained. The characteristic spectral lines of analyzed elements were selected according to the selecting principle of line in LIBS and the calibration curves of major alloying elements Mn, Si and Cr were constructed. The degrees of linear fitting were all more than 0.925 and the limit of detection of Mn, Si and Cr were 75.7, 23.8 and 724.5 μg·g-1, respectively.