To improve the analytical precision of laser-induced breakdown spectroscopy (LIBS), a modified internal standard approach is studied based on the correlation of analytical lines and reference lines. The approach is investigated in detail by principle analysis and numerical calculation firstly, then experimental verification is carried out using the analytical lines of Co I 350.23 nm and Cr I 427.48 nm from the micro-alloy steel standard samples. The results show that, to improve the analytical precision of LIBS, it must meet the necessary conditions that the correlation coefficient of analytical line and reference line γ >0.5, the noise proportional factor b>0.5, and γb > 0.5 . The precision enhancement factor R increases with the increasing of γ , but increases first then decrease with b. When the fluctuation of analytical lines is greater than that of reference lines, the degree of precision improvement decrease gradually.