The efficient separation of acetylene gas (C₂H₂) from C₂H₂/C₂H₄ and C₂H₂/CH₄ is of great significance in various applications due to its high commercial value and numerous practical production scenarios. In this study, a series of ultra-microporous metal-organic frameworks (MOFs) (JLU-MOF161−164) were prepared using mixed-ligand strategy. Drawing upon the principles of isoreticular chemistry, the precise control over ultra-microporous MOFs at a sub-1 Å scale was achieved through functionalization of ligands. The pore sizes decreased from 5.70 to 4.90, 4.24, and 3.74 Å for JLU-MOF161−164. This ultra-fine tuning in pore size significantly improved the selectivity towards C₂H₂ for the series compounds. The C₂H₂ adsorption capacity of JLU-MOF164 reached 76.1 cm³·g⁻¹ at 298 K. Notably, the selectivities for C₂H₂/C₂H₄ (50/50) and C₂H₂/CH₄ (50/50) achieved remarkable values of 10.1 and 716.7, respectively. Compared with JLU-MOF161, these selectivities were enhanced by 3.6-fold and 9.9-fold, respectively. Breakthrough experiments further validated the outstanding dynamic separation performance and excellent cycling stability of JLU-MOF164, highlighting its significant potential for industrial application.