Accurate determination of gas content in shale is of great significance to the evaluation and the exploitation of shale gas reservoirs. At present, however, domestic and foreign scholars have different opinions on the model selection, adsorption mode and adsorption characteristics parameter in the isothermal adsorption experiment of shale methane, and the high-pressure isothermal methane adsorption characteristics are less researched. In this paper, the structural characteristics of the pores in the shale of Lower Silurian Longmaxi Formation were analyzed by conducting N2/CO2 low-pressure isothermal adsorption experiments. Then, gravimetric high-pressure isothermal methane adsorption experiments were carried out, and three different adsorption models (SDR, Langmuir and BET) were used to calculate the content of adsorbed methane. Finally, the methane adsorption characteristics of the samples were researched. And the following research results were obtained. First, as for the pores with a size of 0–50 nm, the distribution of specific surface area and pore volume is bimodal and trimodal, respectively. Compared with mesopores, micropores are higher in specific surface area and lower in pore volume and heterogeneity (D1<D2). Second, among the SDR, Langmuir and BET models, the average error of the first two models is both less than 6%. And methane molecules coexist in shale pores mainly in the adsorption form of monomolelcular layers and micropore filling. Third, in the case of high pressure and great burial depth, temperature is the main factor affecting the adsorption capacity and density of adsorbed methane. In the meantime, the density of the absorbed methane is also affected by thermodynamic parameter, pore structure and heterogeneity. Fourth, methane molecules are mainly adsorbed in the micropores (0.4–0.8 nm) with higher absorption energy and larger specific area in the form of monomolecular layer in a low-pressure phase, and then most of them are adsorbed in micropores and mesopores (1.4–8.0 nm) in the coexistence form of micropore filling and monomolecular layer. In the high-pressure phase, few methane molecules are absorbed in mesopores and macropores in the pattern of multi-molecular layers.