Carbon exchange is a key process that affects the carbon sink capacity of grassland ecosystems and is very sensitive to climate warming. The Tibetan Plateau is covered by large areas of alpine steppe, which is the main carbon pool and fundamental in regional carbon cycling. The present study investigated the effects of warming on carbon exchange in an alpine steppe ecosystem in Baingoin County, Tibetan Plateau from 2012 to 2014. The results showed that there are interannual differences in the effects of warming on the components of carbon exchange in alpine steppes, but there is a negative effect on carbon exchange as a whole. Three-year average results showed that warming significantly reduced the above-ground biomass, gross ecosystem productivity (GEP), ecosystem respiration (ER), and net ecosystem carbon exchange (NEE) in the alpine steppe (P < 0.05), with an average decrease of 15.1%, 36.8%, 19.2%, and 51.5%, respectively. Three-year average soil respiration (SR) did not change significantly (P > 0.05) compared with the control under warming conditions, but warming in 2013 significantly reduced SR (P < 0.05), by 18.1%. Warming has a certain role in promoting the ratio of SR to ER, with the highest increase reaching 40.0%. GEP, ER, SR, and NEE had no significant correlation with soil temperature and soil moisture (P > 0.05), whereas GEP, ER, and NEE showed a significant negative correlation with air temperature (P < 0.05). A decrease in drought stress and above-ground biomass caused by warming were the main factors for the decrease of NEE in the alpine steppe. In summary, our study showed that global warming may reduce the carbon sink capacity of alpine steppes in the Tibetan Plateau.