Clarifying the grazing effects on the carbon sink function on the Tibetan Plateau is highly important under climate change. Based on parallel carbon flux and meteorological observations in nearby grazing and grazing-excluded alpine meadows, the effects of grazing were studied. The results indicated that gross primary productivity (GPP) was dominated by soil temperature (TS) and soil water content (SWC) in both the grazing and grazing-excluded meadows. During the whole growing season, in both meadows, TS exerted the strongest control effects on GPP, whereas the dominant factor shifted to SWC during the peak growing season. During the early and late stages of the growing season, the dominant factor for GPP shifted from SWC to TS in the grazing meadow, whereas the dominant factor remained SWC in the grazing-excluded meadow. The grazing-excluded meadow showed improved temperature conditions, resulting in a lower sensitivity of GPP to TS. In contrast, the grazing meadow showed better water conditions, leading to a lower sensitivity of GPP to SWC, which might result in stronger resistance to droughts. Proper grazing could improve GPP and shift the sensitivity of GPP to environmental factors, providing implications for the sustainable management of alpine meadows.