A fully coupled thermal-hydraulic-mechanical (THM) model was developed to investigate the underlying response mechanisms during heat extraction in fractured geothermal reservoirs. The random fracture network in the stimulated zone was reproduced based on the fractal theory. The coupled model accounts for the dominant physical phenomena including (a) fluid flow, heat transport, and solid deformation in porous media and fractures; (b) local thermal nonequilibrium (LTNE) between rock matrix and flowing fluid; and (c) temperature-dependent fluid thermodynamic properties and stress-dependent pore and fracture permeability. The proposed model was validated by several analytic solutions. Sequentially, the evolution of pore pressure, equivalent temperature, effective stress, and reservoir permeability was analyzed. The sensitivity of the heat extraction performance to fracture network morphology was discussed. Results show that interconnected large-scale fractures dominate mass and heat transport. Widely distributed small-scale fractures contribute to the cooling of the heat rock mass along many flow paths in parallel. The change in effective stress associated with fully coupled thermo-poroelastic effects may induce fracture shear dilation and pore expansion, resulting in an enhancement of the overall reservoir permeability. There is a significant temperature difference between the solid phase and the fluid phase in fractures, but not in porous media. It is more reasonable to use the LTNE theory to analyze the heat extraction of fractured geothermal reservoirs. The fracture network morphology has a profound effect on heat extraction efficiency and injectivity. Undeveloped fracture networks will result in a higher injection pressure, earlier thermal breakthrough, and shorter lifetime, but higher heat extraction ratio. Properly increasing the fracture density can delay the thermal breakthrough time, prolong the service life, and improve the injectivity. Fully connected fracture networks may result in thermal short-circuiting and earlier thermal breakthrough. Generating a complex and scattered fracture network but without preferential channels is conducive to extracting more heat from geothermal reservoirs.