The impact of slickwater on methane adsorption in shale reservoirs is a critical factor in hydraulic fracturing design and production prediction, but its quantitative evaluation is a common problem facing current research. In this work, an advanced integrated model was established to combine multi-scale shale characterization with statistical physics modeling to reveal its underlying mechanisms. Two typical shale samples from the Bohai Bay Basin were taken in situ and treated with different concentrations of slickwater. The evolution of elemental and mineral composition, surface chemistry, and pore structure of shale samples before and after processing was compared using multi-scale characterization techniques. An advanced statistical physical adsorption model was established to analyze the CH4 adsorption characteristics of raw and treated shale. The results indicated that shale treated with slickwater reduced the clay mineral content, specific surface area, and micropore volume, but increased the average pore size. The methane adsorption capacity of shale treated with slickwater was significantly reduced, and CH4 molecules were adsorbed on the shale surface in a mixed parallel or vertical manner. In addition, in the shale samples treated with slippery water, the spontaneous decrease in CH4 adsorption and the increase in disorder resulted in a reduction in the number of low-energy and high-energy sites available for CH4 adsorption on the shale surface. These findings establish a quantitative, mechanistic framework for evaluating slickwater–shale interactions, providing critical theoretical support for enhancing shale gas recovery strategies.