Observing the isotopic evolution of snow meltwater helps in understanding the process of snow melting but remains a challenge to acquire in the field. In this study, we monitored the melting of two snowpacks near Baishui Glacier No. 1, a typical temperate glacier on the southeastern Tibetan Plateau. We employed a physically based isotope model (PBIM) to calculate the isotopic composition of meltwater draining from natural snowpacks. The initial condition of the PBIM was revised to account for natural conditions, i.e., the initial δ 18 O stratigraphy of snow layers before melting. Simulations revealed that the initial heterogeneity of δ 18 O in snow layers as well as ice-liquid isotopic exchange were responsible for most variations of δ 18 O in snow meltwater, whereas new snow and wind drift could result in sudden changes of the isotopic composition of the meltwater. The fraction of ice involved in the isotopic exchange (f) was the most sensitive parameter for the model output. The initial δ 18 O in the snowpack is mirrored in meltwater in case of small f and is smoothed with a large exchange fraction f. The other unknown parameter of the PBIM is the dimensionless rate constant of isotopic exchange, which depends on water percolation and initial snow depth. The successful application of the PBIM in the field might not only be useful for understanding snow melting process but might also provide the possibility of predicting the isotopic composition of snow meltwater and improve the accuracy of hydrograph separation. Plain Language Summary Understanding the process of snow melting is vital for water resources management and protection in the mountainous regions of the Tibetan Plateau. Variations of stable isotopes of oxygen and hydrogen in snow meltwater might give important insights into the process of snow melting. Here we observed and simulated the isotopic composition of snow meltwater of two snowpacks near Baishui Glacier No. 1, which is on the southeastern Tibetan Plateau. The simulations indicate that the isotopic heterogeneity in snow layers as well as ice-liquid isotopic exchange accounts for most of the isotopic variations in meltwater. The successful simulation of isotopic evolution in meltwater provides the possibility of simulating the isotopic composition in natural snow meltwater, which is important for hydrograph separation and paleoclimate studies.