Abstract: A theoretical model of U-tube heat exchangers for a ground source heat pump (GSHP) system was established by taking into account the specific heat of backfill grouting and the vertical heat convection of flow inside a U tube. The influence of the parametric, such as the thermal properties of backfill grouting, the arm distance and the diameter of a U-tube, upon the efficiency of the thermal conversion of a U-tube was analyzed. The calculated results show that the heat cutting-out between the two arms of the U-tube would occur when they are closed. The bigger of the distance of the arms and the diameter of the U-tube, the higher efficiency of the thermal conversion would be obtained. The specific heat of the backfill grouting can influence the efficiency of the thermal conversion of the U-tube in a shorter operation time period, which could be ignore in a longer operation time period. The thermal conductivity of backfill grouting could influence the efficiency of the U-tube significantly. When the thermal conductivity of backfill grouting is bigger than that of soil, the efficiency of a U-tube would increase rapidly. When it is less than the thermal conductivity of soil, the change will become slow due to the cutting-out of heat.