Separation of carbon dioxide (CO2) from a CO2/ethylene (C2H4) mixture represents an important and challenging field in industrial applications. Selective adsorption has become an attractive option as an energy-saving and environmentally friendly CO2 capture technology. In this work, an ultramicroporous metal-organic framework (MOF) Qc-5-Cu was prepared by solution method and applied for the highly selective separation of CO2 from an C2H4 stream. The performance of Qc-S-Cu was evaluated by static and dynamic adsorption experiments. It was found that the single-component adsorption capacities of CO2 and C2H4 on Qc-S-Cu at 298 K and 1 bar were 2.48 and 0.74 mmol/g, respectively. The measured breakthrough curves further illustrate that Qc-5-Cu could effectively separate CO2 from the C2H4 stream for more than 10 cycles. The pore size of the obtained Qc-S-Cu was similar to 3.3 angstrom, as characterized by nitrogen adsorption. The ultramicroporous structure of Qc-5-Cu benefits the separation of CO2, as evidenced by the selectivity values of the CO2 /C2H4 binary mixture (1:99, v/v) based on the ideal adsorbed solution theory (S-IAST approximate to 39.95 avg.), which are larger than some MOFs with open metal sites, owing to the weak interaction between organic ligands with high aromaticity and C2H4, as well as the porous screening effect of Qc-5-Cu on ethylene. The observed high selectivity, excellent stability, and regeneration performance demonstrate that Qc-5-Cu is a promising adsorbent for the separation of the CO2/C2H4 mixture.