In order to enhance the spectral characteristics of the laser-induced breakdown spectroscopy (LIBS), the spectral signals of soil samples with different concentrations of KCl and KI additives were analyzed. The optimal laser energy, the lens to sample distance (LTSD) and the delay time were determined. Under the optimum experimental conditions, the spectral intensity, signal-to-noise ratio (SNR) and plasma characteristics of Cd I 288.08 nm and Al I 394.40 nm in soil samples with 0%, 5%, 10%, 15%, 20%, 25% and 30% KCl and KI additives were analyzed. The experimental results show that the spectral intensities of Cd and Al in soil samples with 15% KCl are 1.33 times and 1.61 times of that of the original sample, while those with 15% KI are 1.84 times and 1.81 times. The SNR, plasma temperature and electron density are improved. This provides an experimental reference for improving the accuracy of LIBS in soil detection.