The physical and mathematical model of three dimensional unsteady coupled heat transfer in the ground heat exchanger is proposed, and corresponding numerical simulations for actual ground heat exchanger and operation condition are carried out. The computed results are compared with those of the thermal resistance test of the ground heat exchanger, which proves the correctness of the model and corresponding numerical methods. Then, the effects of thermal conductivity and specific heat of the ground backfill material on the ground heat exchanger are investigated, which provides theoretical guidance for the thermal response test and the engineering design of the ground heat exchanger.