In-situ analysis or on-line detection is a major advantage of laser-induced breakdown spectroscopy (LIBS) technology. However, in the outdoor environment, people cannot uniformly pre-process samples, then it is difficult to ensure the detection accuracy when facing the various types of samples. In this paper, a multi-line calibration method is proposed to solve the above problem, that is, the calibration curve is established by calculating the intensity ratio of multiple analytical lines and the internal standard element lines, which can reduce the error caused by spectral signal fluctuation and improve linear correlation and detection accuracy. In this experiment, the lead brass alloy samples were taken as an example. The quantitative detection of Pb elements in lead brass samples with different thicknesses (the maximum variation is 2 mm) was carried out by LIBS, and the traditional calibration method and multi-line calibration method were used respectively to establish the calibration curves. The experiment found that for irregular samples, the detection accuracy of the traditional calibration method is very poor, which has no obvious linear relationship from the calibration curve. When the internal calibration method of a single line is adopted, the linear correlation of the calibration curve is greatly improved, and the adjusted determination coefficient reaches 0.724 89. While using the multi-line calibration method (considering the sum of the intensities of multiple analytical lines), it is found that the adjusted determination coefficient of the calibration curve reached 0.984 6 when five Pb lines (Pb 261.42 nm, Pb 280.20 nm, Pb 368.35 nm, Pb 405.78 nm and Pb 520.14 nm) were selected. It can be seen that this method eliminates the spectral intensity fluctuation error caused by sample irregularity and significantly improves the measurement accuracy. While increasing the number of analytical lines can further increase linear correlation, but it also increases the computational complexity, so it is important to choose the appropriate analytical lines. In addition, the multi-line calibration method can also eliminate the matrix effects and spectral interference to a certain extent, which is a simple, effective and universal data processing method. Of course, this method also has limitations (such as extremely heterogeneous sample, extremely irregular sample surface which results in the laser energy below the breakdown threshold, etc.), but by adjusting and optimizing the detection device scheme (for example, increasing laser energy, increasing the diameter of focus spot, using a long-focus lens, etc.), we can improve the advantages of this method. This research content of this paper can provide a new idea for the application of LIBS in-situ analysis and on-line detection.