The two-year operating performance and the variation in the underground thermal environment of a ground source heat pump (GSHP) system are presented in this work; the GSHP has been installed in an office building in Hangzhou. The system consists of two heat-pump units, each with a heating/cooling capacity of 470 kW/465 kW, and 201 boreholes. The flow rates and temperatures of water on the user and buried-pipes side, the electrical power consumption of the heat-pump units and the water pumps, and the underground temperatures were monitored during this project. The result of the performance monitoring revealed that the average daily COP of the units and the system was approximately 5.0 and 3.0, respectively, during cooling seasons, and 4.5 and 2.7, respectively, during the heating seasons. It was found that the electricity consumption of the water pump accounted for 39.7% of the total electricity consumption, which is high enough to degrade the system performance. It was also found that the heat released by the GSHP system during the cooling season is more than the heat absorbed during the heating season at the hot -summer and cold -winter zone. The underground heat imbalance rate was approximately 24% in the first year, and was approximately 53% in the second year. The result of underground temperature monitoring showed that the underground temperature increased by 1.1 degrees C after two years of system operation. This reveals that the heat exchange between the GSHP5 and the soil is unbalanced, which eventually caused "heat accumulation". Moreover, the variation in the underground thermal environment was revealed by the changes in underground temperature according to time, depth, and season. Finally, in this work, a simple model is presented, which quantitatively verifies the underground heat imbalance through the underground temperature variation. Reasonable agreement of results verified the underground heat imbalance, and confirmed the reliability of the model. Overall, this study provides a significant amount of experimental data for the analysis of the underground thermal environment variation and the heat balance. The results of this study will contribute toward the development of more creative methods for the mitigation of the impact of "heat accumulation" on the environment at the hot -summer and cold -winter climate zone. In addition, the method used in this work can be applied to other climate zones for the study of the thermal effect of the GSHP ground heat exchanger. (C) 2017 Elsevier Ltd. All rights reserved.