Marine organic-rich shale in South China has considerable exploration potential, and the shale adsorption capacity has a great impact on the accumulation of shale gas. To study the methane adsorption capacity of marine shale, ten shale samples from the Lower Cambrian Qiongzhusi Formation in eastern Yunnan province were investigated by organic geochemical analysis (total organic carbon content, thermal maturity, and kerogen type), X-ray diffraction (XRD) analysis, field emission scanning electron microscopy (FE-SEM), and low-pressure nitrogen adsorption and methane adsorption experiments. Based on the different adsorption mechanisms of various pores, the Dubinin-Radushkevich and Langmuir-Freundlich models were used to construct a supercritical adsorption model of shale. Combined with this model, the mechanisms and characteristics of shale adsorption under supercritical conditions were analyzed. The maximum absolute methane adsorption capacities of micropores (V-1) and mesopores-macropores (172) were also calculated. Not only have the relationships between organic geochemistry, mineral compositions, pore structure parameters, and maximum absolute methane adsorption capacity (both V-1 and V-2) been discussed but the impact of moisture on the methane adsorption capacity of shale has also been investigated. The results show that the maximum adsorption capacity of mesopores macropores is greater than that of micropores. Both V-1 and V-2 are positively correlated with the TOC content, and V-2 is more correlated with the TOC content than V-1. High maturity is not conducive to methane adsorption of shale. The maximum absolute methane adsorption amounts of per-unit organic matter have positive correlations with the clay mineral content, but show negative correlations with the quartz content. Different clay minerals have different methane adsorption capacities. Both V-1 and V-2 increase with increasing specific surface area. V-1 has a positive correlation with the micropore volume, but V-2 has no apparent relationship with the mesopore-macropore volume. Moreover, shale samples with higher moisture contents have lower methane adsorption capacity. It is anticipated that the results of this study will provide guidance for the adsorption characteristics and influence factors of high maturity marine shale.