A microporous and robust bimetal-organic framework [Cd₂Ba(NH₂-BTB)₂]·2(DMA)·2(H₂O) (1) was constructed by mixing the heterometallic nodes and an tridentate carboxyl ligand with amino group, benefiting from the synergistic effect of active sites, exposed amino groups and ultramicroporous structure. This compound displays an extraordinary selectivity of CO₂/CH₄ and C₂H₂/CH₄ (16.7 and 146.3) at 298 K and 1 atm, which can exceed those of many reported MOFs under the same conditions. This work provides an important model to design metal-organic frameworks for the adsorption and separation of small gas molecules.