The soil samples at six different humidities were prepared for studying the influence of soil humidity on laser-induced plasmas. By analyzing the characteristic spectral lines of Pb with the wavelength of 405.78 nm, it was demonstrated that the intensities of spectral line, SNR and RSD are linearly inversely proportional to the soil humidity. Under the approximation of local thermal equilibrium, by analyzing the characteristic spectral lines of Fe within the wavelength range of 400 to 440 nm, the temperature of plasmas was obtained by means of the two-dimensional Boltzmann plane. The experimental results show that the plasma temperature monotonously changes from 11 800 to 7 800 K and the electron density monotonously changes from 3.3×106 cm-3 to 2.8×106 cm-3, when the humidity increases from 0 to 20%.