Abstract: Cement mortar is prone to product defects during of pouring, curing and loading, which will significantly change its heat conducting properties. The defects in cement mortar were divided into two basic types of cavities and cracks according to its external characteristics and effects. Based on the interaction direct deduction (IDD) estimation method of mesomechanics theory, the prediction models of equivalent thermal conductivity of dry cement mortar with both cavities and cracks alone or in combination were established. Based on the inverse method of numerical simulation results, the random distribution influence function of the cracks was proposed, which improved the prediction accuracy of the proposed model. The IDD calculation results were compared with the numerical simulation results to verify its rationality and accuracy. The results show that the prediction model fully reflects the influence of different contents and characteristics of cavities and cracks on the equivalent thermal conductivity of cement mortar. The model has high prediction accuracy. The proposed random distribution influence function of cracks has high accuracy and good applicability. In the case of a certain defect rate, the effect of cavities on the equivalent thermal conductivity of cement mortar is more significant than that of cracks. The prediction model has a clear physical basis and a simple form, which is convenient for engineering applications.