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δ13C and δ15N are extensively used to understand the biogeochemical mechanisms that moderate ecosystem carbon (C) and nitrogen (N) processes. Little is known about the responses of δ13C and δ15N to alpine grassland degradation on the Qinghai‐Tibetan Plateau (QTP), which prevents a full understanding of degradation‐induced changes in C and N cycling there. We investigated the vegetation δ13C and δ15N, soil δ13C and δ15N, soil properties, and plant community composition of alpine grassland on the QTP that were in different states of degradation. Our results show that the vegetation δ13C and δ15N, and soil δ13C and δ15N, increased with the severity of degradation, whereas soil organic carbon (SOC) and total N content decreased as degradation became more severe. The aboveground biomass percentage of forbs was positively correlated with the soil C/N ratio, vegetation δ13C, and soil δ13C, and accounted for the largest proportion of the variance for both vegetation δ13C and soil δ13C (17.25 and 23.65%, respectively). The vegetation δ15N and soil δ15N were negatively correlated with the soil C/N ratio, which explained the largest proportion of the variance (18.01 and 25.81%, respectively). Our results suggest that C cycling is strongly moderated by plant community composition, because forbs species and C4 species, were more prevalent in degraded alpine grassland. Meanwhile, N cycling is indirectly regulated by changes in community composition via its effect on the soil C/N as the degradation became more severe for alpine grassland on the QTP. [ABSTRACT FROM AUTHOR]