Ground heat exchanger (GHE) is an important component of ground coupled heat pump system (GCHP). To calculate the soil temperature around GHE accurately and fast, a refined response factor model (RF model) is proposed. It combines the heat transfer inside and outside the U pipe through the temperature of pipe wall and the heat flux of U pipe. For the RF model, after calculating the response factors by CFD simulation, the soil temperature can be calculated by the deduced analytical equations. The sandbox experiment is built up to validate the the RF model. Based on the experiment, this case is also studied by the numerical simulation and the RF model. Results show that the soil temperature differences between the RF model and the experiment are only -0.21°C ~0.69°C at the 96th time step. The relative errors of the soil temperatures between RF model and numerical simulation at the 1800th time step are only 1.86%~3.94%. RF model consumes 30% time of the numerical simulation for the soil temperature calculation with 1800 time steps and consumes only 1% time of the numerical simulation for that with 350400 time steps. Therefore, the RF model is accurate and fast to calculate the soil temperature around the GHE with fluid inside.