The practical application of spectral sensors for molten steel composition online detection has remained limited due to the combined effects of extreme temperature, slag erosion, and transmission challenges associated with weak emissions from trace elements. In this study, a molten steel composition sensor was developed based on Laser-Induced Breakdown Spectroscopy (LIBS). The sensor incorporates a 1.5-meter-long multi-layered probe lance through which the laser beam is focused onto molten steel at the lower end, where the emitted plasma is analyzed to enable online composition measurement. During operation, the probe lance penetrates through the slag layer and maintains immersion in molten steel at a certain depth withstanding temperature exceeding 1650°C. Both the probe lance and the entire optical return path are purged with argon gas to enhance the signal. Through the acquisition of optimal parameters, the industrial application was first performed? using the LIBS sensor? during the refining of 120-ton molten steel. Test set demonstrates R2 of 0.9361, 0.9643, 0.9809, and 0.9815 with corresponding RMSE of 0.04wt.%, 0.03wt.%, 0.03wt.%, and 0.07wt.% for C, Mn, Si, and Cr respectively. Results indicate that the LIBS sensor has preliminarily achieved in-situ online detection of specific elements in molten steel, showing promising application potential.