Abstract: The dielectric constant model is the relationship between dielectric constant and volumetric water content. Using proper model, the water content can be predicted by the dielectric constant from GPR. The water content of soil is closely related to its physical properties in engineering. The engineering status of soil can be classified by its liquid limit and plastic limit. Therefore, the engineering status of soil may be judged by the dielectric constant directly based on the study on different soils, which can make geotechnical design be easier. The relationship between dielectric constant and volumetric water content is discussed based on a series of loess samples with different sand contents. The applicability of several models is analyzed according to the engineering properties of loess. Moreover, a dielectric constant model, which has clear meaning and concise form, is put forward. It is suggested that the soil should be classified according to the index of sensitive rate of dielectric constant to change of moisture. The results agree with the experimental data. Furthermore, the dielectric constant of loess at liquid limit and plastic limit and the expression for liquidity index and dielectric constant are given. The method to evaluate the engineering status of soil is proposed according to the dielectric constant.