Combination fracturing technique with alternating injection of high-and low-viscosity fracturing fluids balances fracture height and fracture network complexity, making it an effective strategy for enhancing the stimulated reservoir volume (SRV) of shale oil reservoirs. However, existing research on fracturing fluid viscosity has primarily focused on fracture pressure and morphology, while the effects on imbibition and hydration have been insufficiently explored. This study combines macro-and micro-scale experiments to systematically compare changes in water absorption, fracture development, pore structure, mineral composition, and mechanical properties of shale before and after exposure to slickwater with varying viscosities. Results show that deionized water hydration significantly alters shale's physical properties, generating pronounced dissolution pores and a large number of hydration-induced fractures parallel to bedding planes, leading to a severe deterioration in mechanical properties. In contrast, as slickwater viscosity increases, the hydration effects weaken, evidenced by the absence of obvious dissolution pores, minimal changes in existing fracture widths, and no new hydration fractures. The degradation of mechanical parameters, such as elastic modulus, was less pronounced than with deionized water. The analysis suggests that the higher interfacial tension of deionized water facilitates its penetration into clay mineral interlayers, promoting clay swelling, inducing new fractures, and accelerating pyrite oxidation and carbonate dissolution, further enhancing hydration via the formation of dissolution pores. In contrast, the polyacrylamide coating formed by slickwater on the shale surface inhibits mineral dissolution and pore generation through hydrogen bonding and surface complexation. This result reveals the impact mechanism of slickwater viscosity on shale hydration and provides theoretical guidance for the design and optimization of fracturing fluids in the field.