Chemical fixation of CO₂ with propargylic amines/alcohols into high value-added fine chemicals represents promising and practical routes for CO₂ resource utilization, but it remains a great challenge to achieve low-concentration CO₂ conversion in both reactions through one single catalytic system. Herein, a robust copper cluster [(C₆H₁₅NH)(Cu₈L₈I)] (I@Cu₈) with nanosized cavities has been constructed. Importantly, the I@Cu₈ cluster as a nanoreactor can efficiently catalyze the chemical conversion of both propargylic amines and propargylic alcohols with CO₂ at simulated flue gas concentrations, due to the unique substrate enrichment effect of its well-defined nanocavities. Catalytic mechanism investigation and DFT calculations illustrate that the excellent catalytic performance originates from the simultaneous activation of alkynyl (─C≡CH) bonds and amino/hydroxyl (─NHR/─OH) groups in the I@Cu₈ catalytic system, in which the I@Cu₈-substrate intermediates have been confirmed through ESI-MS analysis. Impressively, this is the first example of a cluster-based nanoreactor catalyst for low-concentration CO₂ cyclization reactions with both propargylic amines and propargylic alcohols. This work provides a foundation for the construction of metal cluster-based catalysts for CO₂ conversion into valuable chemicals.