Using modified Dobson dielectric constant model as the basic model, we analyze and validate the relationships between soil volumetric water content or soil salinity and dielectric constant. On this basis, we analyze the relationship between backscattering coefficient and dielectric constant by extracting the backscattering coefficient of the radar image at different polarization modes. The results show that the volumetric water content and soil salinity are the main influencing factors of dielectric constant of the soil. The volumetric water content is the decisive factor of the real part of the dielectric constant, and it determines the value of the real part of the dielectric constant. The imaginary part of the dielectric constant is affected by various factors, and the soil salinity is the main factor. The interaction between volumetric water content and soil salinity affects the backscattering coefficient. At the cross polarization mode, the dielectric constant is the main factor affecting the backscattering coefficient of the radar image. The dielectric constant of the soil has a certain potential in monitoring the soil salinity and volumetric water content. It is entirely possible to retrieve soil salinity by radar images.