A copper-based metal-organic framework, [Cu2(PBTDA)(H2O)2] (UTSA-98, UTSA = the University of Texas at San Antonio; H4PBTDA = 5',5''''-(1,4-phenylene)bis([1,1':3',1″-terphenyl]-4,4″-dicarboxylic acid)), has been solvothermally synthesized. The alternative connection of classical dicopper secondary building units and deprotonated four-branched PBTDA4- ligands led to the formation of the three-dimensional framework of UTSA-98 with one-dimensional rhombic channels. Its guest-free phase, UTSA-98a, uptakes much more C2H2 (82.6 cm3/g) than CO2 (40.3 cm3/g) at 298 K and 100 kPa, resulting in a high adsorption selectivity of 5.2. Furthermore, the efficient separation ability of UTSA-98a toward the C2H2/CO2 gas mixture was further verified by laboratory-scale fixed-bed breakthrough experiments.