Abstract <p>Fracturing fluid viscosity is a critical engineering factor in determining the morphology of hydraulic fractures in shale reservoirs. The effect of alternating high- and low-viscosity fluid injection on fracture propagation in laminated shale remains unclear, particularly under in-situ stress configurations where bedding planes are parallel to the injection borehole and perpendicular to the maximum principal stress. This study conducts true triaxial hydraulic fracturing experiments on laminated shale outcrops under such a configuration, alternating injections of high- and low-viscosity fracturing fluids. Meanwhile, the dynamic fracturing behavior and fractures information within the specimens are determined using acoustic emission (AE) system and high-energy CT scanning technology, respectively. The results demonstrate that high-viscosity fracturing fluid can achieve rapid pressure buildup during the fracturing process, facilitating the hydraulic fractures to penetrate bedding planes. The use of low-viscosity fracturing fluids helps activate some of the bedding planes and natural fractures. Combining high- and low-viscosity fracturing fluids is beneficial for creating complex fracture networks. The effectiveness of hydraulic fracturing with equal proportions of high- and low-viscosity fracturing fluids exceeds that of other proportions. The findings provide field-applicable guidelines for laminated shale reservoirs with analogous in-situ conditions, recommending equal alternating ratios of high- and low-viscosity fluids to optimize network complexity.</p>