Calibration- free laser induced breakdown spectroscopy (CF- LIBS) requires that the elements be normalized, various elements are involved in calculation. However, the spectral line of microelements in the soil is weak, and it is difficult to draw the Saha-Boltzmann plot. The element particle ratio method is used to calibrate the concentration of Cr in soils from a variety of national standard samples as well as local acquisition. The temperature of the plasma is calibrated from Saha-Boltzmann equation and the electron density is determined from the line width of Al I 309.284 nm. The assumption of the local thermal equilibrium (LTE) is validated by the McWhirter criterion. The particle ratio of Cr and Si is calculated by Saha-Boltzmann plot and Saha equation. The concentration of Cr can be calculated. The relative error for the national standard soil samples is less than 7%, and is less than 16.438% for the field collected samples. The results indicate that the element particle ratio method can be used in the concentration analysis of Cr in soils to improve performance of the LIBS technique in rapid detection of soil element content.