Middle and deep geothermal resources are abundant and have the advantages of being clean, stable, and reliable. However, a widely recognised theoretical model of deep ground heat exchangers is currently lacking owing to the uncertainty of the underground geotechnical environment and the complex heat transfer mechanism of such heat exchangers. Therefore, considering the stratification of the rock and soil layers, a heat extraction model of a coaxial deep borehole heat exchanger was established in this study. In addition, MATLAB software was used to conduct an unsteady-state numerical simulation based on the finite difference method, and the simulation results is verified through a long-duration comparison with actual measured engineering data. It is observed that (1) the numerical calculation results are in agreement with the actual engineering measurement results; (2) under the test conditions, the radius of influence of the deep borehole heat exchanger on the temperature of the surrounding rock and soil layer is approximately 7 m after continuous operation throughout the heating season; (3) the heat extraction increases with increasing circulation flow rate and outer diameter of the buried pipe; and (4) the linear meter heat exchange power should not be greater than 150 W. This study provides important guidance for research on coaxial deep borehole heat exchangers. (C) 2021 Elsevier Ltd. All rights reserved.