We developed a fully coupled thermo-hydro-mechanical (THM) model for hydraulic fracturing treatments, which was verified via comparison to published literature. This model was then utilized to investigate complex fracture network propagation in naturally fractured reservoirs. Mixed finite volume and finite element methods were used to solve this coupled system, in which the MPFA L-method was employed to evaluate the fluid-heat, inter-element flux because of the existence of full tensor permeability caused by imbedded fractures. To elucidate the crossing behaviors as hydraulic fractures meet inclined natural fractures, we extended Gu et al.'s (2011) crossing criterion to a 3D scenario. Moreover, we integrated the modified Barton et al.'s model (Barton et al., 1985) proposed by Asadollahi (2009) to address the shear dilation process, which is helpful for accurately estimating the dynamical fracture aperture responding to changes of stresses, fluid pressure, and temperature. Numerical experiments showed that principal factors controlling the complexity of the induced fracture network included stress anisotropy, injection rate, natural fracture distribution (fracture dip angle, strike angle, spacing, density, and length), fracture filling properties (the degree of cementation and permeability), fracture surface properties (cohesion and friction angle), and tensile strength of intact rock. We found that: (1) the lower the injection rate and the smaller the stress anisotropy, the more complex fracture network the could be developed; (2) high tensile strength could increase the possibility of the occurrence of shear fractures; and (3) under the conditions of large permeability of fracture-filling combined with small cohesive strength and friction coefficient, shear slip could become the dominant mechanism for generating a complex fracture network. The model developed and the results presented can be used to understand the propagation of complex fracture networks and aid in the design and optimization of hydraulic fracturing treatments.