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Under Rayleigh equilibrium condition, stable isotopic ratio in residual water increases with the decrease of the residual water proportion f exponentially, and the fractionation rate of stable isotopes is inversely proportional to temperature. However, under kinetic evaporation condition, the fractionation of stable isotopes is not only related to the phase temperature but also influenced by the atmospheric humidity and the mass exchange between liquid and vapor phases. The ratio δ in residual water will not change with f after undergoing evaporation of a long time for great relative humidity.The rate that the evaporating water body reaches isotopic steady state is mainly dependent on the relative humidity in atmosphere. The analysis shows that the actual mean linear variety rates, about -30.0, of the δ18O in residual water versus the residual water proportion at Nagqu and Amdo stations are consistent with the simulated process under temperature of 20 ℃ and relative humidity of 50%.The distillation line simulated under Rayleigh equilibrium condition is analogous to the global meteoric water line (MWL) as the temperature is about 20 ℃. Under non-equilibrium condition, the slope and constant values of distillation line are directly proportional to temperature and relative humidity.According to the basic data, the simulated distillation line is very consistent with the actual distillation line of Qinghai Lake.