Laser-induced breakdown spectroscopy has showngreat-potential for application in the online analysis of molten steel composition. However, detecting the important C, P, and S component elements in molten steel has been challenging because their effective emission lines cannot be transmitted over long distances in the air. This paper studies the influence of the argon environment on detecting C, P, and S elements in steel under a 1.5-meter long probe gun; it is found that an argon flow rate that is too large or too small is not conducive to spectral measurement. When the gas flow rate is set to 11 L·min^-1, the spectrum obtained is the most stable, and the spectral line intensity is the largest. Using 14 standard steel samples, the three elements C, P, and S were detected and quantitatively analyzed under optimal gas flow. After the internal standard calibration, the limit of detection (LOD) of the three elements were 0.009%, 0.04%, and 0.015%, the relative standard deviations (RSD) were 2.34%, 1.05%, and 1.01%, and the correlation coefficients (R) were 0.998, 0.997 and 0.987, respectively. The root mean square errors (RMSE) were 0.02%, 0.02% and 0.03%, respectively. The research results of this paper verify the effectiveness of the design of the probe gun and provide an important design basis for the online analysis of C, P, and S in the composition of molten steel.