The surfaces of soil with different moisture contents had been surveyed by the spectral polarimetry in the mid-wave infrared range. The results indicate that the thermal radiation from the water-bearing soil surface is almost unpolarized, and the polarization characteristic of the soil surface is mainly caused by the reflection of solar light. Especially in the strong sunshine spectral range, the degree of polarization increases monotonically with the increase of soil moisture. This quantitative relationship provides basis for soil moisture monitoring by the remote sensing of the polarization of light in the mid-wave infrared range.