The Qinghai-Tibet Plateau has the highest and largest alpine grassland globally, facing growing impacts from accelerated warming and intense wet-dry alternation. Current studies indicate that water stress is a major driver of vegetation growth suppression. However, there remains limited understanding of how the sensitivity of alpine grassland productivity to vapor pressure deficit (VPD) and root zone soil moisture (SM) varies spatially and its main determinants. Here, we quantified the sensitivity of vegetation productivity to VPD and SM on the QinghaiTibet Plateau based on long-term satellite observations of net primary productivity (NPP) and gridded meteorological data (1982-2021), and identified the primary factors which affect drought sensitivity across different alpine grassland types. Our results showed a significant increase in atmospheric aridity and soil dryness over the past 40 years, which has led to widespread negative effects on NPP. Among grassland types, the greatest sensitivity to VPD was observed in alpine steppes, with alpine deserts being most responsive to SM. Mean annual temperature and precipitation were important variables contributing to the geographic variation in grassland drought sensitivity. Interestingly, NPP was negatively related to VPD, except in regions with low background VPD values (<1.8 hPa). A more pronounced negative correlation between NPP and VPD was observed at higher VPD values when VPD was below a threshold of 7.2 hPa. Additionally, low SM was found to exacerbate this negative correlation, with the most significant effects occurring in semi-arid regions. This study represents one of the initial investigations into the spatial patterns of NPP sensitivity to VPD and soil water condition, as well as their specific threshold effects in alpine grasslands.