A novel robust three-dimensional octahedral-like caged metal-organic framework with ultrahigh selectivities has been demonstrated to exhibit the potential for purification of methane in nearly pure form from a 6-component gas mixture at room temperature. Display omitted •An ultramicroporous MOF based on octahedral-like cages with dual functionalities was successfully designed and constructed.•The elaborate pore size/confinement made the MOF has high adsorption capacities and strong affinities for C2/C3 hydrocarbons.•The MOF represents high selectivities for C2/CH4 and C3/CH4 mixtures.•One-step CH4 purification from a highly complex six-component C1/C2/C3 hydrocarbons mixture was realized using the MOF. Given the crucial significance of using cleaner methane (CH4) to replace else fossil fuels in remitting energy consumption and preventing environmental degradation, developing prominent adsorbents to purify CH4 from multicomponent mixtures is fundamentally important but faces great challenges. Cage-based metal-organic frameworks (MOFs) bring about widespread attention in solving numerous separation problems due to their inherent structural preponderances. Herein, we constructed an octahedral-like cages-based MOF (NUM-18) that incorporates two different types of cages within the whole framework and bears abundant Lewis basic sites (naked N atoms) and nonpolar aromatic rings immobilized in the pore surface. Benefitting from its intriguing structural characteristics, the pure-component gas adsorption properties were systematically investigated and indicate that NUN-18a (activated NUM-18) has excellent adsorptive capacities with respect to C2-C3 hydrocarbons than CH4. The IAST adsorption selectivities for C2/CH4 are above 14.0, while the adsorption selectivities of C3/CH4 startlingly surpass 86.0, respectively, all of which forebode that NUM-18a can achieve efficient recovery of C2-C3 hydrocarbons associated with CH4 purification. Furthermore, the actual separation feasibility for an equimolar 6-component C1/C2/C3 hydrocarbons mixture was examined by simulated dynamic column breakthrough experiments. Finally, molecular simulation calculations were adopted to ascertain potential gas adsorption mechanisms. This work provided a new paradigm for developing novel porous crystalline MOFs materials to separate hyper-complex gas mixtures.