Abstract: To provide a theory for the optimized design of a U-tube ground heat exchanger used in ground source heat pumps, CFD software was used to simulate the processes of flow and heat transfer in the U-tube ground heat exchanger. The results show that the heat transfer efficiency of the U-tube ground heat exchanger increases with the increasing distance between two tubes and with the increasing heat conductivity of backfilling soil. The effect of the distance between two tubes and the heat conductivity of backfilling soil on the U-tube ground heat exchanger's efficiency are a very complex nonlinear relation. When the hole-depth is deeper than 80 m, the temperature rise ratio of two tubes increases rapidly and the heat losing between two tubes increases. In practical operation, the hole-depth is better not to be too deep.