A laser induced breakdown spectroscopy (LIBS) system has many experimental key parameter factors. Focusing position in laser induced breakdown spectroscopy (LIBS) system is an experimental key parameter factor. The optimization of spectral intensity has been studied, but the results are often inaccurate. In this Letter, A quantitative analysis internal standard method was proposed to optimize the parameter of LIBS experimental system. One 100mm focusing lens and eight standard alloy steels with rich element content were employed as samples. By employing internal standard method, the minority elements (Mn, Cr, Cu) in steels were quantitatively analyzed and the element detection sensitivity was obtained. The limit of detection (LOD) of the minority elements under different focusing states was calculated, and finally the optimal focus position was obtained. The result which indirectly shows that the best laser focusing position of LIBS system with 100mm focusing lens is 6mm below the surface of samples, and the limit of detection (LOD) of the minority elements maintain the minimum value at this position (0.00159 wt.%), which indicates that the quantitative analysis method is effective and reliable for optimizing the parameters of LIBS system. © 2022 SPIE