The calibration-free (CF) method is a quantification approach for laser-induced breakdown spectroscopy (LIBS) that enables the content determination of detected elements without a calibration curve. However, the actual quantitative results of traditional CF-LIBS often exhibit discrepancies with respect to the real values. In order to improve the accuracy of CF-LIBS quantitative analysis results, three standard Cu-Zn alloy targets with different mass ratios were used for the calculation of CF-LIBS quantitative analysis, and accurate quantification of the Zn/Cu quality ratio in the alloys was successfully achieved through utilization of "quasi-optically thin state" and "self-absorption correction". In the implementation, we firstly approached the theoretical intensity ratio of copper lines (I521.8 nm / I515.3 nm) by adjusting laser energies and detection delays, so as to find the "quasi-optically thin state" of the plasma. And then, under such a plasma state, the spectral intensity was well corrected by the self-absorption via referencing the corresponding coefficient. Based on the proposed method, the Cu/Zn quality ratio was accurately determined using CF-LIBS, exhibiting excellent agreement with the actual content. The quantitative biases of the three standard copper-zinc alloy targets were observed all less than 3.5%, indicating high precision of measurements. It proves that the developed method is effective in improving alloys quantitation of LIBS, and more applications are expected in other fields in the future.