The elemental composition of Cr, Si and Mn of molten steel is on-line monitored by laser-induced breakdown spectroscopy (LIBS). Solid steel is melted in medium frequency induction furnace into liquid, and raw materials are added in the furnace to adjust the contents of Cr, Si and Mn. Based on laser-induced breakdown spectroscopy, an experimental equipment named SIA-LIBS-02 is developed and used to measure the components of molten steel. The effect of the steel temperature and component concentration on spectral line intensity is studied. The results show that the steel temperature strongly affects the intensity of different characteristic lines, the relationships between temperature and line intensity are different for different elements, and the detection capability of minor elements in molten steel is higher that in normal steel. In addition, relative line intensity can resist dynamic disturbance in on-line measuring, and successfully monitor the concentration changing of component Cr, Si and Mn in the case of constant steel temperature.