Vegetation photosynthesis dynamics and their drought responses are fundamental for understanding ecological stability in arid/semiarid regions. In this study, we integrated hourly multispectral data from geostationary satellites with Orbiting Carbon Observatory-3 sunlight-induced chlorophyll fluorescence (SIF) observations to explore the intradaily course of vegetation photosynthesis over the Qilian Mountains. We further examined how drought conditions, represented by the standardized precipitation evapotranspiration index (SPEI), modulated the intradaily photosynthesis curves. The results indicate that: 1) SIF exhibits distinct spatial gradients in the Qilian Mountains. In eastern grasslands, SIF increases in June and peaks in August, while in western bare/sparsely vegetated areas, it remains low during the growing season; 2) SPEI analysis shows greater drought severity in the western than eastern Qilian Mountains during 2020-2022, with abrupt moderate-to-extreme droughts in the central-northern parts in Jun. 2022; 3) during the drought period, the intradaily photosynthesis curve systematically shift compared with the normal period, and the intradaily peak photosynthesis time are almost the same during the drought and normal period; and 4) among major vegetation types, croplands have the highest peak SIF and greatest fluctuations, forests exhibited the most stability, whereas grasslands were most sensitive to drought. The findings demonstrate strong spatiotemporal coupling between SIF variations and moisture conditions, and provide new evidence of how drought stress regulates intradaily photosynthesis in the Qilian Mountains.