The complex fracture networks produced by hydraulic fracturing are crucial to the higher gas/oil production capacity from tight reservoirs. In this study，the physical simulation experiments on hydraulic fracturing were conducted by a true triaxial test system with the oil shale samples from the Triassic Yanchang Formation，and 3D CT reconstruction was performed on the fracture networks of the samples before and after fracturing. The complexity of the fracture networks were quantitatively described on the basis of the fractal theory and the digital image processing technology. The results indicate that the change rate of the fractal dimension of fracture networks are 0.45%-3.64% before and after fracturing based on the cube covering method. A negative correlation is always between the change rates of the fractal dimension and the horizontal stress ratio. Moreover，the change rate of the fractal dimension grows first and then declines as the fracturing fluid viscosity varies，and it reaches the maximum at the fluid viscosity of 5.0 mPa·s. A low horizontal stress ratio and a low fluid viscosity can intensify the complicated morphology of fracture networks，while over low or high fracturing fluid viscosity restrains the formation of fracture networks.