When a Borehole Heat Exchanger (BHE) array is coupled with heat pump to provide cooling and heating to the buildings, thermal interaction between BHEs may occur in the subsurface. In the long term, imbalanced seasonal thermal load may lead to low or high temperature zones accumulating in the centre of the array. In this study, numerical models are configured according to a real BHE array project in Leicester, UK, and verified against monitoring data. Based on this reference model, a series of numerical experiments are conducted to investigate the response of circulation fluid temperature to different settings of imbalanced thermal load. It is found that over long-term operation, the sub array with a larger number of installed BHEs is shifting its thermal load towards the other branch with less BHEs installed. Within each sub array, the heat injection rate on the central BHEs is gradually shifted towards those located at the edge. A linear correlation is also found between the working fluid temperature increment and the amount of the accumulated heat injected into the subsurface.