The energy performance of a ground-source heat pump (GSHP) system will gradually deteriorate and the annual operation cost will increase annually if the heat injected into and extracted from the underground soil is imbalanced. To solve the underground heat balance problem and improve the energy efficiency of the system in the long-term, a temperature difference control strategy for the GSHP system coupled with borehole free cooling (BFC) is proposed in this study. An office building located in Tianjin is selected as a case study. According to the established simulation models by Designbuilder and TRNSYS, measured data is used for validation. The new control strategy and two other contrasting control strategies are validated with TRNSYS. The results show that free cooling with borehole water matches the indoor air parameter with the design indoor conditions and also improves the average annual cooling efficiency of the GSHP system to 16.22. Moreover, the underground soil heat imbalance can be improved, and the average annual energy consumption and operation cost can be decreased to 55.9% and 56.0%, respectively. The proposed control strategy has the potential to improve heat imbalance and reduce energy consumption and operation cost for GSHP systems. (C) 2017 Elsevier B.V. All rights reserved.