The ground source heat pump (GSHP) system has attracted attention, because of its stability of heat production, and the high efficiency of the system. However, there are few studies on the prediction method of the heat exchange rate for a horizontal GSHP system. In this research, in order to predict the performance of a horizontal GSHP system, coupled simulation with a ground heat transfer model and a heat exchanger circulation model was developed, and calculation of heat exchange rate was conducted by the developed tool. In order to optimally design the horizontal GSHP system, the flow rate of circulation water, and the depth and buried spaces of heat exchangers were considered by the case study. As a result, the temperature of circulation water and the heat exchange rate of the system were calculated in each case.