Field simulation experiments were conducted to identify the water source of the wet dune in Alxa Desert. We simulated 59 mm of precipitation, and found that the maximum infiltration depth in the dune profiles was only 46 cm, suggesting that the rainfall infiltration through sand to the groundwater is nearly impossible in this area. In the four dune profiles, δD, δ18
O, and Cl-
concentrations in the soil water, and the moisture contents of the wet dune at different depths, were analyzed. In Alxa Desert with strong evaporation, groundwater in the form of film water evaporates, condensates, transports to the surface, and finally discharges by evaporation. The isotopic compositions of springs, wells, lakes, and soil water show that they all originate from the groundwater, whose source is exogenous. We suggest that deep-circulation water channels may exist in Zaduo-Yabrai and the Langshan-Shigatse basement fault zone, and water leakage from the Qinghai-Tibet Plateau may be the main recharge source of the groundwater in Alxa Desert.