Removal of CH4 and CO2 from C2H2 streams remains challenging in the chemical industry. Herein, a robust three-dimensional metal-organic framework, Cu-CPAH, was designed and synthesized through a hydrothermal method. Cu-CPAH exhibits highly selective C2H2 adsorption capacity with respect to both CH4 and CO2, which is ascribed to the enrichment of active sites in the framework. Dynamic breakthrough results reveal that Cu-CPAH serves as a solid adsorbent for high-efficiency purification of C2H2 from an equal proportion of C2H2/CO2 or C2H2/CO2/CH4 at room temperature. Discrete Fourier transform simulations confirm that various active sites preferentially interact with C2H2 other than CO2 and CH4, signifying for the first time that the imino functional groups in the cage contribute greatly to the preferential affinity to C2H2 over CO2 and CH4.