Abstract: The thermal conductivity of unsaturated soil, one of the key parameters, is very important for analyzing the thermal phenomenon in geotechnical engineering. Based on the distribution features of three-phase system in soil, through analogy the heat propagation in the earth is regarded as the seepage in soil. A parallel combination model for heat conduction in the soil is generalized. A mathematical model for thermal conductivity of the unsaturated soil is derived, and the model parameters are analyzed. To some specific soil, the porosity has dramatical influence on the thermal conductivity, but the influence of saturation is not significant. Finally, by comparing the predicted value of thermal conductivity to the data measured by KD₂ instrument, the maximum error range is less than 10%. The results show that the proposed model can be employed to predict the thermal conductivity of the soil.