In recent years, the ground source heat pump (GSHP) system has been widely used due to its high unit efficiency, considerable energy conservation and low operating cost. However, the heat transfer efficiency of ground heat exchangers (GHE) in some projects decreases year by year. This results in the decrease of performance of the GHSP system. This is mainly because of lacking of the deep research about the heat and moisture transfer influence on the GHE in unsaturated soil. In this paper, a new model for predicting the soil thermal conductivity under different temperatures was developed. By comparing with the Campbell and de V-1 models, the new model showed better performance on predicting the soil thermal conductivity. Thereafter, a coupled heat and moisture transfer model in unsaturated soil was established for the GHE. In addition, the model was verified by comparing its results against those established by other researchers. By the results, it was found that the model calculation results agreed well with the experimental data in the literature. Finally, the effects of different factors, including the soil types, soil porosity on the characteristics of the GHE were studied. (C) 2016 Elsevier B.V. All rights reserved.