Unconventional reservoirs are gaining more and more attention in recently years. Pore pressure and temperature variations in unconventional reservoirs during stimulation are of great importance in reservoir exploration and development. Fluid injection is one of the most commonly used permeability enhancement technique, which induces significant changes in the stress, pore pressure and temperature fields of reservoir rock. Coupled thermo-hydro-mechanical (THM) processes are involved in those changes. In this work, three-dimensional finite element methods (FEM) are developed and applied to simulate the response of unconventional reservoirs to fluid injection. The FEM model is assisted with continuum damage mechanics and stochastic fracture network model, which simulates the intact rock failure process and natural fracture networks deformation during stimulation, respectively. Numerical simulations using the current model are present in this dissertation, and applied to analyzing different aspect of reservoir response to fluid injection, such as, wellbore instability, intact rock failure, natural fracture deformation, permeability enhancement, and injection induced micro-seismicity. The results indicate the important roles of rock heterogeneity and natural fractures’ distribution in influencing the stimulation effect. The correlation among damaged zone, permeability enhanced volume, and induced MEQ distribution is shown in the results. The influences of injection plan, boundary conditions, and in-situ stress states on the stimulation results are also illustrated in the modeling examples.