The water saturation in shale reservoirs plays a key role in controlling shale oil accumulation and production. The microscale distribution of pore water constrains both the content and spatial occurrence of adsorbed and free oil. Therefore, investigating the occurrence characteristics of pore water in shale oil reservoirs is essential for understanding the mechanisms of oil–water interactions. In this work, the shales from the Qingshankou Formation in the Gulong Sag were selected. Water vapor adsorption experiments were conducted under controlled relative humidities of 23%, 33%, 56%, 75%, 85%, and 98%. At each relative humidity (RH), low-temperature nitrogen adsorption tests were conducted to monitor the evolution of pore structure during water vapor adsorption. Subsequently, changes in pore-scale heterogeneity and the factors controlling water adsorption were analyzed. The results show that the Dent model effectively distinguishes between primary adsorption, occurring at specific adsorption sites, and secondary adsorption among water molecules, corresponding to Type Ⅰ and Type Ⅲ isotherms, respectively. At low RH, water vapor adsorption is dominated by primary adsorption, forming a monolayer. As RH increases, secondary adsorption becomes dominant, resulting in multilayer formation, and at high RH, capillary condensation prevails. Nitrogen adsorption data indicated that increasing RH reduced the BET specific surface area (SSA) and pore volume (PV), whereas average pore diameter (APD) increased. Multifractal parameters ΔD and D−10−D0 decreased with increasing RH. At low RH, larger micro- and mesopore volumes were more strongly affected by water vapor adsorption, leading to more uniform pore filling and reduced heterogeneity. In contrast, the D0−D10 increased with increasing RH, especially when RH exceeded 85%, reflecting the influence of capillary condensation within macropores. Clay mineral content significantly influenced adsorption, particularly under low RH. TOC exhibited a similar trend, likely due to organo-clay complex pore-fractures. PV and BET SSA similarly influenced adsorption, with notable effects mainly at low RH. These insights are expected to elucidate the microscale occurrence mechanisms of pore water within the complex pore–fracture system of shale, and provide a basis for understanding oil–water distribution and interactions in shale oil reservoirs.