In order to improve the spectral intensity and the signal-to-back ratio of the characteristic spectral lines, promote the application of LIBS technology in the detection of trace heavy metals in soil. The experimental parameters in the process of soil analysis were optimized, and the element of Cr was analyzed. The Nd∶YAG laser with an output wavelength of 1 064 nm, the pulse width of 10 ns and pulse frequency of 1~10 Hz was used as the light source to focus the pulse laser on the surface of soil samples to generate laser plasma. Experimental parameters such as laser excitation energy, sample distance from lens and spectrometer collection delay were optimized. Firstly, the spectral intensity and the signal-to-back ratio of the laser energy from 60 to 110 mJ were compared. It was found that the plasma radiation intensity rises first and decreases, and the best experimental results can be obtained when 90 mJ excitation energy is selected. Secondly, the variation of spectral intensity from 5 mm before coke to 5 mm after coke is compared. It was found that when the distance between the sample and the lens was 1 mm after the focus (i. e. the focus position was 121 mm), the characteristic spectral lines and the information to back ratio of Cr elements reached the best. Finally, the influence of the acquisition delay of the spectrometer on the spectral line strength and the signal-to-back ratio were analyzed. The results show that the influence trend of energy on plasma radiation intensity is roughly the same, and the experiment result is best when the collection delay is 1 000 ns. Under the optimum experimental conditions (that is, the laser energy 90 mJ, focus position 121 mm, the acquisition delay 1 000 ns), 12 soil samples containing heavy metal Cr were detected by spectroscopy. Meanwhile, in order to reduce the interference of the external environment, the average values of the spectra obtained from 10 laser ablation positions of the same sample were pretreated. Chromium (Ⅰ) 357.86 nm, chromium (Ⅰ) 425.44 nm and chromium (Ⅰ) 427.49 nm were selected as characteristic lines. The calibration curves of doping concentration and spectral intensity were established. The detection limits of the three lines were 74.62, 64.07 and 67.49 mg·kg-1, respectively. The goodness-of-fit values R2 were 0.98, 0.97 and 0.99, respectively. RMSE was 0.41, 0.33 and 0.35, respectively. At the same time, partial least square method and support vector machine algorithm are introduced to improve the accuracy of calibration model further. The results show that the optimization of experimental parameters improves the quantitative detection parameters of trace elements by LIBS technology. The optimal spectral intensity and signal-to-back ratio are obtained. Good experimental results are obtained by the Lorenz fitting calculation of calibration curve, which has important reference significance for the detection of trace heavy metal elements by LIBS technology.