The thermal probe method for measuring the thermal conductivity of 4 red clays in Guangxi was used to obtain the relationship between the thermal conductivity and water content of red clay. Then the common calculation models was had simulation analysis with the Composite Medium Theory in order to select out the appropriate calculation models which can be used to predicted the thermal conductivity of red clay effectively. On this basis，the calculation value of solid thermal conductivity of red clay in those calculation models was discussed according to the mineral composition analysis. The results show that the thermal conductivity of the red clays increases with the increasing of volumetric water content at the same dry density and the critical water content is a demarcation point. The increasing rates are different in these two sections. It is found that the thermal conductivity of red clay can be calculated into two subsections by Effective Medium Theory(EMT) model and Parallel model. The thermal conductivity was predicted by the EMT model when the soil water content is lower than the critical value，and by the Parallel model when the soil water content is higher than the critical value. Finally，the method for taking-value of solid thermal conductivity of red clay with Difference Effective Medium Theory have a comprehensive consideration about the influence of mineral compositions of red clay on the solid thermal conductivity，and the calculation results are much more approximate to the practice.