To investigate the influence of environmental relative humidity variations on water adsorption and desorption in the shallow surface layer of earthen sites, isothermal moisture adsorption-desorption tests were carried out on earthen site soils with four distinct dry densities under seven levels of relative humidity by using the saturated salt solution method. The test results demonstrate that with the increase in relative humidity, the rates of moisture adsorption and desorption gradually decrease, while the time required to reach equilibrium state progressively increases. The equilibrium moisture contents of isothermal adsorption and desorption of earthen site soils both rise with increasing relative humidity. Specifically, the adsorption equilibrium moisture content decreases with increasing dry density, whereas the desorption equilibrium moisture content increases with increasing dry density, showing a pronounced hysteresis effect. The isothermal adsorption and desorption curves of earthen site soils both exhibit an obvious two-stage behavior, consisting of a rapid stage and a slow stage of moisture transfer. Through comparative analysis, it is found that the Peleg model and Fengchi model are suitable for fitting the isothermal moisture adsorption-desorption curves of earthen site soils. In addition, the Parallel Exponential Kinetics(PEK) model can well characterize the isothermal moisture adsorption-desorption process, and the first 50% of the equilibrium time data can be used to accurately predict both the time to reach final equilibrium and the final equilibrium moisture content.