The Tibetan Plateau, characterized by its high altitude and extensive area, is one of the most prototypical alpine regions in the world. Exploring the characteristics and determinants of carbon stable isotope composition (δ¹³C) in plants and soil across its alpine grasslands holds significant implications for comprehensively understanding carbon cycling processes within this unique ecosystem. Our study entailed the collection and measurement of carbon stable isotope natural abundances of plants and soils from 135 grassland sites across the Tibetan Plateau. The δ¹³C characteristics of different plant functional groups and surface soil (0-10 cm) were measured, and the relationships between δ¹³C abundances and climate, edaphic factors were determined. The results showed that: (1) The δ¹³C in Forb was significantly lower than that in Poaceae, Cyperaceae and Fabaceae (P ＜ 0.05). The δ¹³C of surface soil was significantly positively correlated with that in Poaceae, Cyperaceae and Fabaceae (P ＜ 0.05), but not with that in Forb. The sensitivity of surface soil δ¹³C to the three plant functional groups followed the order of Poaceae > Fabaceae > Cyperaceae. (2) Analysis of environmental factors influencing δ¹³C in Poaceae, Cyperaceae, Fabaceae, and surface soil indicated that a greater relative contribution of climate factors over edaphic factors, with solar radiation exerting the most substantial influence among climatic factors. Notably, no significant correlation was detected between the δ¹³C of Forb and climatic, edaphic factors. Our results also underscored solar radiation as a pivotal determinant of the δ¹³C characteristics in plants and surface soil. These findings can provide valuable empirical data and theoretical insights for the δ¹³C characteristics and organic carbon dynamic cycle of plants and soil in alpine grassland ecosystems.