Increase in atmospheric dryness, characterized as vapor pressure deficit (VPD), might constrain terrestrial productivity. Nevertheless, the precise temporal impacts of VPD on the gross primary productivity (GPP) of alpine wetland ecosystems during the growing season remain elusive. The alpine ecosystems of the Tibetan Plateau (TP), where productivity is highly constrained by the cold climate, have experienced pronounced warming of 0.26 degrees C decade-1 with associated increase in VPD. In this study, by examining eddy covariance observations taken in an alpine wetland on the TP over five consecutive years, we characterized when and how VPD variation causes negative impact on ecosystem productivity. The TP alpine wetland functioned as a net CO2 sink with magnitude of 164.6 +/- 22.0 g C m-2 yr-1. It was found that VPD played a crucial role in the seasonal variation in GPP especially in the peak growing season, that is, it even suppressed the positive effect of temperature on GPP. As temperatures declined in the latter stages of the growing season, the inhibitory effect of VPD on GPP gradually diminished. We further found that the VPD at midday (13:00-14:30) was crucial for inhibition of photosynthesis and midday depression of GPP. Our results emphasize the role of atmospheric dryness during the middle growing season and at midday on GPP, thereby providing new insights into how VPD affects CO2 uptake in a warming climate.