Deep borehole ground source heat pump (DBGSHP) is a new type of heat pump heating system which extracts deep geothermal energy through heat exchange and can be applied for space heating in winter. To date, the development of deep borehole heat exchangers (BHEs) is limited to the cognized structure design and there is a lack of the experimental studies. This paper presents the investigation of the heat transfer characteristics of the heat exchanger of a DBGSHP heating system through both field test and numerical simulation. A field test was first carried out based on the DBGSHP implemented in a demonstration project. A numerical model was then developed to facilitate the evaluation of the heat extraction capacity and the outlet temperature of the coaxial deep BHEs. Based on the numerical model developed, a sensitivity study was further performed to examine the effect of the primary parameters including the inlet velocity, inlet temperature, flow pattern (one was that the circulating fluid flowed from the inner pipe to the annular space and the other was that the circulating fluid flowed from the annular space to the inner pipe) and pipe diameter on the performance of deep BHE. The results from the field test indicated that the average heat transfer capacity of each single borehole, the average COP of the heat pump unit and the DBGSHP heating system COP were 286.4 kW, 6.4 and 4.6, respectively. The simulation results matched well with the field test data, and showed that the inlet fluid velocity between 0.3 m/s and 0.7 m/s as well as the circulating fluid flowed from the annular space to the inner pipe can result in a better performance for the system of concern. The results from this study could be used as a reference basis for optimal design of coaxial deep BHE and to promote the utilization of deep geothermal energy.