In order to clarify the influence of fracture dilatancy on the exploitation lifetime and heat extraction performance of deep fractured metamorphic geothermal reservoirs, a thermo-hydro-mechanical coupled discrete fracture network model was established for the Yanggao-Tianzhen geothermal field in the Datong Basin, Shanxi Province. The evolution laws of fracture aperture, stress field, seepage field and temperature field under cold-water reinjection were investigated, and the exploitation lifetime and heat extraction performance of the geothermal system were evaluated. The results show that the effective normal stress on fractures is significantly reduced by pore-pressure increase and thermal contraction induced by cold-fluid injection. Shear slip and dilation of major fractures are then promoted, and preferential seepage channels are formed. The migration of cold fluid along the dominant channels is accelerated by fracture dilatancy, and earlier thermal breakthrough is induced. Compared with the case without dilatancy, the effective exploitation lifetime is shortened to about 36 a, with a reduction of 10.0%; the production temperature is reduced to 143.2 ℃, with a reduction of 13.2%; and the instantaneous heat extraction power is decreased to 20.70 MW, which is 15.0% lower than the initial value. It is concluded that fracture dilatancy is a key factor controlling the long-term heat production capacity and exploitation lifetime of deep fractured metamorphic geothermal reservoirs. A theoretical basis can be provided for the operational regulation and development evaluation of similar geothermal systems.