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Diversity lithofacies in lacustrine shale possess different pore structures with different shale gas adsorption mechanisms. It is of great significance for lacustrine shale gas reserve prediction to clarify the adsorption mechanism of methane in lacustrine shales. A series of experiments was carried out on core samples from the Upper Triassic Xujiahe Formation in the western Sichuan Basin of China. CO 2 adsorption, N 2 adsorption, and scanning electron microscopy experiments were performed to analyze the pore structures of lacustrine shale. The pore structure of siliceous shale is more complex with higher values of pore fractal dimensions (D 1 and D 2 ) than other lithofacies, while, based on fitting curves to methane isothermal adsorption data, the method of the corrected Akaike's information criterion (AICc) measuring the goodness of the nonlinear fitting curves, pore structure, and analysis of methane adsorption layers are combined to evaluate and select methane adsorption model in this work. Additionally, the methane adsorption process could be described by a two combined first-order rate (TCFOR) model of adsorption rate. As shown by TCFOR in most samples, the normalized adsorption capacities of the fast process higher than that of the slow process (Q 1 > Q 2 ) appear during adsorption experiment. Thus, methane may not enter the micropores in large quantities under low experimental pressure (<10 MPa). The adsorption mechanism of methane in most samples is monolayer adsorption (Langmuir + Henry model) with the lower AICc value than other adsorption models. The DA + Henry model is suitable for the process of methane absorption in which the Q 2 > Q 1 appeared early in the methane adsorption experiment. Occasionally, the phenomenon of Q 2 > Q 1 could be found in the siliceous shale owing to the micropores dominating in it. Ultimately, the relationship between accumulated dV/dD (reflecting the density of different pore sizes) from N 2 adsorption experiment and the absolute adsorption amount (V abs ) calculated by models is figured out to determine the adsorption model of methane under different pore sizes. Methane adsorption in pores smaller than 3.4 nm is mainly a micropore filling mode, and filling of pores between 9.6 and 50 nm is mainly a monolayer mode. Nevertheless, the adsorption of methane in the pores between 3.4 and 9.6 nm is a transition from monolayer adsorption to micropore filling. The results enhance our understanding of the methane adsorption mechanisms in different lithofacies with different pore structures.