Understanding vulnerability of terrestrial ecosystems and underlying mechanisms are fundaments for decision-making on the adaptation and mitigation of global climate change impacts. Vulnerability of ecosystem could be quantified by the sensitivity and adaptability of productivity to climatic changes. In this study, a method was developed to quantify the sensitivity, adaptability and vulnerability for alpine ecosystems. The method respectively quantified the vulnerability, sensitivity and adaptability of ecosystems through the remote sensing-based actual net primary productivity (NPP_R) and the climate-driven potential net primary productivity (NPP_C) over Qinghai province from 1982 to 2018. Then the spatial and temporal changes were explored in ecosystem vulnerability and the underlying mechanisms were analyzed in the terms of the effects from human activities and climate change respectively. The results showed that:(1) the NPP_R- and NPP_C-based vulnerability of the ecosystems showed a spatial pattern as moderately vulnerable, vulnerability increases from southeast to northwest in descending order from not vulnerable to extremely vulnerable. (2) Cropland had lower vulnerability with the NPP_R- and NPP_C- based index of -1.31 and -0.93, due to its higher adaptation level and lower sensitivity level, followed by forest with the index of -1.18 (NPP_R) and -1.06 (NPP_C), and grassland with the index of -0.17 (NPP_R) and -0.17 (NPP_C), while desert had higher vulnerability level with the index of 0.77 (NPP_R) and 0.78 (NPP_C), due to higher sensitivity level and lower adaptation level. (3) The NPP_R-based vulnerability of alpine grassland was found having the two thresholds (-2.2±0.8)℃ and (5.5±0.8)℃ of temperature, the one precipitation threshold (387±45.6) mm, therefore the two aridity index thresholds of (78.2±20.2) and (14.2 ±20.2). And the same thresholds were found for the NPP_C-based vulnerability, which has the similar value as the NPP_R-based vulnerability. The thresholds indicate that under optimal climate conditions, ecosystems will have higher adaptation and lower sensitivity, that is, lower vulnerability. But it will be higher vulnerability if temperatures are very cold or very warm, or the precipitation are very low. (4) The human activities exert strong impact on the vulnerability of the ecosystems over the eastern regions though the impacts were smaller in the term of the regional average for the ecosystems in whole province. This study demonstrated that vulnerability is spatially vary but dependent on climate condition in alpine fragile ecosystems, which should be considered as a theoretical fundament in the decision-making on ecological protection. Further the thresholds found in this study would provide a case study in ecosystem ecology and should be widely explored over the fragile ecosystems worldwide.