Abstract: The water adsorption performance of shale gas reservoirs is a very important factor affecting their gas in place (GIP) contents, but the water-holding capacity and mechanism of over-mature shale, especially organic pores, are still not fully understood. In this study, systematic water vapor adsorption (WVA) experiments were carried out on the Lower Cambrian over-mature shale and its kerogen from the Sichuan Basin, China to characterize their WVA behaviors, and combined with the low-pressure gas (N2 and CO2) adsorption experiments, the main influencing factors of WVA capacity of the shale and the absorbed-water distribution in its organic and inorganic nanopores were investigated. The results show that the WVA isotherms of shale and kerogen are all type Ⅱ, with an obvious hysteresis loop in the multilayer adsorption range, and that the positive relationship of the shale TOC content with the WVA capacity (including total adsorption capacity, primary adsorption capacity and secondary adsorption capacity) and WVA hysteresis index (AHIW), and the greater adsorption capacity and AHIW of kerogen than the shale, all indicate that the hydrophilicity of organic matter (OM) in the over-mature shale was underestimated in previous research. Although both the shale OM and clay minerals have a significant positive effect on the WVA, the former has a stronger adsorption ability than the latter. The WVA capacity of the studied Lower Cambrian shale is significantly greater than that of the Longmaxi shale reported in literatures, which was believed to be mainly attributed to its higher maturity, with a significant graphitization of OM. The shale micropores and non-micropores play an important role in WVA, especially OM pores. There are primary and secondary adsorption for water vapor in both the micropores and non-micropores of OM, while these adsorptions of minerals mainly occur in their non-micropores. These results have important guides for understanding the gas storage mechanism and exploration and development potential of marine over-mature shale in southern China, especially the Lower Cambrian shale.