Water is one of the causes of softening of soft rocks in deep strata. Water absorption in soft rock with high clay content may cause strength softening and deformation. In this paper, soft rock samples from three deep strata are investigated, and the stratigraphic characteristics such as stratum physical parameters, mineral composition, and pore structure are systematically analyzed. The characteristics of water vapor adsorption curves of deep strata soft rock are obtained by water vapor adsorption experiments. The authors analyzed the main factors controlling water vapor adsorption capacity of the rock samples. The experimental results show that the water vapor adsorption capacity increases with increase in humidity. When the humidity exceeds 0.8, the adsorption of water vapor by soft rock increases rapidly. The adsorption capacity of shale is significantly higher than that of conglomerate samples, and the difference between coarse conglomerate and fine conglomerate samples is insignificant. It is shown that water vapor adsorption correlates positively with clay mineral content, particularly minerals with high specific surface area. Under low humidity the temperature has little effect on the adsorption of soft rock water vapor. As the humidity increases, a rise in temperature can significantly increase the water vapor adsorption capacity The results also show that the soft rock water vapor adsorption capacity correlates negatively with initial moisture content, porosity, and permeability. This study is of great significance for understanding the hydrological characteristics and catastrophic processes of deep strata soft rocks.