Abstract: Flow and thermal extraction problem of working fluid entering geothermal reservoirs is a typical thermo-hydro-mechanical（THM） coupled problem. The density of working fluid and the permeability of geothermal reservoirs have evolved under the influence of THM coupling during flow and heat extraction in the thermal reservoir, which leads to changes in the heat transfer efficiency of geothermal reservoirs. In this work, a two-dimensional fully THM coupled FEM model is established to simulate a long term enhanced geothermal system（EGS） thermal extraction process. The evolution pattern of reservoir displacement, pore pressure, temperature is analyzed, and the evolution effect of working fluid density and reservoir permeability on heat transfer efficiency of EGS system is also investigated. The results show that among the factors of temperature, confining pressure and pore pressure, temperature is the most important influential factor on the evolution of fluid density and permeability during stable extraction stage. However, for the final heat transfer efficiency of reservoir, density evolution delays the cooling rate of the reservoir outlet temperature and improves the net heat extraction rate. While permeability evolution accelerates the cooling of the outlet temperature, it also improves the net heat extraction rate.