It is well known that Ground Source Heat Pump (GSHP) systems have the potential for significant energy savings and have demonstrated superior performance compared to conventional HVAC systems. Researchers and engineers have made lots of efforts in the study and improvement of GSHP systems in order to ensure the reliability and high efficiency of this type of system. Nevertheless, unlike other conventional HVAC systems, e.g., a system using chillers and cooling towers, the long-term monitoring and analysis of a GSHP system are significantly essential, especially considering the possible variation of the ground temperatures year after year under various building heating/cooling loads. The change of ground temperatures may substantially affect the system efficiency of a GSHP system and sometimes may even cause a failure of the system in providing enough heating/cooling effect to buildings. Therefore, this paper focuses on the long-term monitoring and simulation of a vertical closed-loop GSHP system used in the cold climate region of the U.S. A TRNSYS simulation model was established, which was calibrated first by using the measurement data and then utilized to forecast the future variation of the ground temperatures. Additionally, the critical design parameters of the GSHP system are identified by conducting a sensitivity analysis. The results show that for this building, the soil thermal conductivity has a significant impact on the ground temperatures compared to other design parameters studied, and changing the borehole separation distance has a greater impact on the ground temperatures during winter than increasing the number of boreholes. The use of different zoning strategies may greatly affect the energy simulation result for an office building with a large number of windows installed on the south walls and equipped with a GSHP system for space heating and cooling.