The PVA/LiCl composite membrane is an important material with a selective permeation to moisture. In this study, the sorption kinetics characteristics of the PVA/LiCl composite membrane are investigated in a dynamic humidity environment. The sorption kinetics curves of the PVA/LiCl composite membranes with different LiCl contents were measured in the range from 10 to 90% relative humidity, and the kinetics characteristics of the sorption components of the total moisture uptake were analyzed with a triple-mode sorption model. The results show that the process of pooling adsorption occurs only when the amount of Henry absorption exceeds a threshold. With the increase in LiCl content, the threshold gradually decreases, indicating that introducing LiCl in the membrane mainly affects the process of pooling adsorption. In the desorption part of the kinetics curve, the amounts of Henry absorption and pooling adsorption gradually decrease, while the moisture adsorbed by the hydrophilic hydroxyls, i.e., the amount of Langmuir adsorption, is still largely retained in the membrane, resulting in the water-retaining performance in a low-humidity environment. It is noteworthy that in a high-humidity environment, the amounts of Henry absorption and Langmuir adsorption are almost evenly distributed in the film, while the amount of pooling adsorption near the surface is significantly higher than that at the center. The reason is that with the increase in humidity, the rate of pooling adsorption near the surface increases significantly, resulting in the majority of the moisture diffusing into the dense membrane clusters near the surface. The established theoretical method and model parameters can be adopted to predict the moisture sorption and desorption processes of the PVA/LiCl composite membrane in a dynamic humidity environment.