Alien plant invasion has caused severe ecological problems in many places around the world. The introduction of exotic species may significantly alter the community composition and has a potential to change water cycle in the local ecosystem. Hydrocotyle vulgaris L. (H. vulgaris) is an introduced species, widely spreading in both aquatic and terrestrial habitats as a potentially invasive plant in humid region of China. A greenhouse experiment was conducted to investigate the impact of H. vulgaris invasion on the evapotranspiration of the native grassland community during the establishment period. Three gradients (none, one, and five seedlings) of the introduced species were established either in bare soil or in an experimental plant community consisting of four grassland species. A bare soil treatment without any plants was conducted as a control. The results showed that H. vulgaris did not affect the evapotranspiration when growing alone in any gradient treatment, while the native grassland community exhibited more rapid evapotranspiration when growing along with H. vulgaris. Moreover, the evapotranspiration of the native grassland community was higher as the introduced species settlers increased in number, but the initial H. vulgaris ramets with different gradient did not significantly affect the growth of native plant community. A reoccurrence experiment was conducted and yielded identical results. The results indicated that H. vulgaris could cause the native grassland community to have higher evapotranspiration, and that such an effect becomes more significant with increasing numbers of invasive propagules. This study may have profound implications for understanding the regional eco-hydrological alteration caused by alien plants invasion.