The moisture transport history to the south of the Tibetan Plateau was modeled using the meteorological data provided by NCEP in this paper, and the modeled results were compared with the measured δ¹⁸O in the south of the Tibetan Plateau. The relation between δ¹⁸O in precipitation in the south of the Tibetan Plateau and moisture trajectories was discussed. The results show that the extremely low δ¹⁸O in precipitation in the south of the Tibetan Plateau is always related to the moisture from the low-level sea surface evaporation. The long-distance transport of moisture also contributes to low δ¹⁸O in precipitation probably due to the rainout process during moisture transport. It is also found that low δ¹⁸O in precipitation is also related with deep layer transport of moisture, and with intensive condensation in the upper layers of the atmosphere, resulting in low δ¹⁸O because of depletion of heavy isotopes in deep condensation. However, high δ¹⁸O in precipitation whether in monsoon period or not is always companied with moisture coming from the upper layers, and the moisture is from northern or western sides of the plateau. The interpretation of the modeled results is in agreement with the isotope fractionation processes.