As one of the most important ways to calculate the plasma temperature when the laser induced plasma is under local thermal equilibrium condition, the use of Boltzmann algorithm is extended. The experiments prove that the soil laser induced plasma is under local thermal equilibrium condition, then the Boltzmann plot is built by several Fe atomic spectral lines. According to the fitting line, the intensity of other Fe spectral lines are calculated and the error between the calculated and the experimental results is lower than 15%. The characteristic spectral lines of Fe, Ti, Ca in soil are abundant and easy to overlap with other spectral lines. The method can assist identifying and separating some unknown spectral lines. By building the Boltzmann plot of Mn, the intensity of 405.89 nm line is calculated and its impact on the Pb 405.78 nm line is studied. The results show that this method can be applied in the field of spectral line identification and separation.