In order to study the adsorption characteristics of water vapor on unsaturated loess, isothermal adsorption experiments under different humidity conditions were carried out by vapor equilibrium method. The adsorption behavior of water vapor on unsaturated loess surface was analyzed, and the effects of temperature, mineral composition and content, dry density on the adsorption property of soil were discussed. The experimental results show that the water vapor adsorption capacity of unsaturated loess increases with the increase of relative humidity, and the whole process includes three stages: monolayer adsorption, multilayer adsorption and capillary condensation. The GAB model can be used to describe the water vapor adsorption process of unsaturated loess. There is a significant negative correlation between water vapor adsorption and temperature. When the relative humidity is constant, the adsorption capacity of water vapor decreases with the increase of temperature. Water vapor adsorption of unsaturated loess is closely related to mineral composition, and clay mineral content directly affects its water vapor adsorption capacity. In addition, the effect of dry density on water vapor adsorption can be divided into two stages. When the relative humidity RH<80%, water vapor adsorption capacity increases with the increase of dry density. Until entering the capillary condensation stage, with the increase of dry density, the amount of water vapor adsorption no longer increases but decreases.