Precise syngas ratio control is crucial for efficiently producing high-quality clean fuels such as naphtha, kerosene, and diesel. In light of the rising demand for energy-efficient hydrogen separation, this study investigates the H₂/CO separation performance of novel benzimidazole-linked polymer (BILP-101x) and poly(p-phenylene benzobisimidazole) (PBDI) membranes fabricated via a simple interfacial polymerization process. The effects of temperature, pressure, and H₂ molar fraction on the membranes' separation performance were comprehensively evaluated. Both membranes displayed high H2 permeance (BILP-101x∼136 GPU; PBDI∼76 GPU) and selectivity (BILP-101x∼78; PBDI∼50) for H2/CO separation at 150 °C. The superior H₂ permeance of BILP-101x was attributed to its higher fractional free volume and diffusion coefficients compared to PBDI, as confirmed by molecular simulations. Notably, both membranes demonstrated remarkable stability during long-term testing under simulated syngas conditions (50/25/25H₂/CO₂/CO, 100 °C for 120 h), outperforming most polymeric membranes reported in literature for H2/CO separation. The superior H2/CO separation performance coupled with the excellent stability (over 120 h) endows BILP-101x and PBDI membranes with an attractive application prospect for industrial syngas ratio adjustment.