Hydraulic fracturing experiments with low-viscosity fluids, such as supercritical CO2, demonstrate the formation of complex fracture networks spread throughout the rocks. To study the influence of viscosity of the fracturing fluids on hydraulic fracture propagation, a hydromechanical-coupled cohesive zone model is proposed for the simulation of mechanical response of rock grains boundary separation. This simulation methodology considers the synergistic effects of unsteady flow in fracture and rock grain deformation induced by hydraulic pressure. The simulation results indicate a tendency of complex fracture propagation with more branches as the viscosity of fracturing fluids decrease, which is in accord with experimental results. The low-viscosity fluid can flow into the microfractures with extremely small aperture and create more shear failed fracture. This study confirms the possibility of effective well stimulations by hydraulic fracturing with low-viscosity fluids.