We present an uncertainty analysis of thermo-hydro-mechanical (THM) coupled processes in a typical hot-dry-rock reservoir in crystalline rock. The conceptual model is an equivalent porous media approach which is adequate for available data from the Urach Spa and Falkenberg sites (Germany). The finite element method (FEM) is used for the numerical analysis or fully coupled THM processes, including thermal water flow, advective-diffusive heat transport, and thermo-poro-elasticity. Reservoir parameters are considered as spatial random variables and their realizations are generated using conditional Gaussian simulation. The results show the influence of parameter ranges on the reservoir evolution during long-term heat extraction taking into account fully coupled THM processes. We found that the most significant factors are permeability and heat capacity variations. The study demonstrates the importance of taking parameter uncertainties into account for geothermal reservoir evaluation in order to assess the predictability of numerical modelling.