During the summer monsoon season, the moisture of precipitation events in southern and central regions of the Tibetan Plateau is mainly moisture from the Indian Ocean transported by the Indian monsoon and terrestrial vapor derived from the surface of the Tibetan Plateau. However, the respective contributions of these two types of moisture are not clear. From June to September, the excess deuterium values of precipitation and river water in the Nam Co basin are higher than those for the southern Tibetan Plateau.This reflects the mixing of evaporation from Nam Co and local atmospheric vapor. On the basis of theory for estimating the　contribution of evaporative vapor from surface water bodies to atmospheric vapor and relative stable isotopes in water bodies (precipitation, river water, atmospheric moisture and lake water), this study preliminarily estimates that the average contribution of evaporation from the Lake Nam Co to local atmospheric vapor has varied from 28.4% to 31.1% during the summer monsoon season in recent years.