Background and aims <p>Elevated CO₂ (eCO₂) typically increases plant photosynthesis and productivity through the CO₂ fertilization effect (CFE), but the response pattern of plant biomass allocation to eCO₂ remain poorly understood, especially across different vegetation types.</p> Method <p>In our study, we analyzed 639 observation pairs from 120 published articles on global CO₂ manipulation experiments through meta-analysis to determine vegetation biomass and its allocation responses to eCO₂.</p> Results <p>eCO₂ significantly enhanced the root-to-shoot ratio (R/S) by 14.25%, indicating a reallocation of resources between root and shoot systems. Belowground biomass (BGB) increased by 36.17%, substantially higher than the 20.43% increase in aboveground biomass (AGB), suggesting preferential carbon allocation to belowground tissues. Additionally, variation in R/S response to eCO₂ were observed across ecosystem types, with stronger positive effects in grasslands (16.17%) than in forests (12.65%). Boosted regression tree analysis identified climate as the dominant factor controlling the response ratio of R/S, accounting for 54.08% of the total variation. In comparison, AGB and BGB increases were greater in forests than in grasslands due to their higher photosynthesis rate and plant water use efficiency under eCO₂ treatment. Furthermore, plant biomass responses were significantly correlated with eCO₂ manipulation magnitude. AGB, BGB, and R/S linearly increased with the magnitude of eCO₂.</p> Conclusions <p>Our findings suggest that rising CO₂ levels could modify terrestrial plant biomass allocation strategies, and terrestrial plants could allocate more biomass to belowground organs, especially in grasslands.</p>