Aims: Phenolics can help plants mitigate stress of adverse environments. Elevated CO2 (eCO2) and arbuscular mycorrhizal fungi (AMF) may improve phenolics synthesis, however, the contribution of eCO2 to phenolics in plants colonized by AMF under heavy metal exposure is still unclear.Methods: We investigated the effect of eCO2 (285 ppm above ambient CO2) on phenolic acids and flavonoids in black locust (Robinia pseudoacacia L.) grown in cadmium (Cd)-contaminated soils under Funneliformis mosseae (FM) colonization using 13C isotope tracer technique.Results: Elevated CO2 significantly enhanced leaf and root Cd by 25.7% and 38.7%, respectively, and reduced total Cd by 36.7% in rhizosphere soils under FM colonization. Total phenolic acids decreased by 48.5% under eCO2 + FM relative to FM colonization. Elevated CO2 significantly decreased gallic acid, chlorogenic acid, p-hydroxybenzoic acid, caffeic acid, p-coumaric acid, ferulic acid, robinin, quercetin, kaempferol, and acacetin by 52.2%, 45.1%, 9.7%, 89.1%, 45.2%, 24.8%, 26.2%, 13.5%, 32.6%, and 34.1% under FM colonization, respectively, and the contribution of + 285 ppm CO2 to these compounds was 0.15, 0.33, 0.74, 0.14, 0.62, 0.38, 0.14, 0.23, 0.10, and 0.17 µg g−1 DW, respectively. The contribution decreased under FM colonization except for to chlorogenic acid, and the contribution was the most to total phenolic acids. Phenylalanine ammonia-lyase and nitrogen significantly affected phenolic acids and flavonoids synthesis.Conclusions: The results could provide some insights into the contribution of eCO2 to plant resistance to heavy metals and plant sequestering heavy metals under AMF colonization.