Developing novel materials for membrane separation is extremely crucial. Benzimidazole-linked polymers (BILPs) show great potential for H₂ purification due to their high thermal and chemical stability, while only a few BILPs have been used for membrane-based gas separation. In this work, the BILP-5 composite membranes were prepared via a facile interfacial polymerization. Dense and smooth BILP-5 composite membranes were prepared on α-Al₂O₃ support and characterized by scanning electron microscope (SEM) and atomic force microscope (AFM). The effects of interfacial polymerization reaction duration, temperature, and pressure on the gas separation performance of BILP-5 membranes were systematically investigated. The resultant BILP-5 membrane shows good stability and a high H₂/CO₂ selectivity of 16 with a H₂ permeance of 362 GPU under mixed gas test at room temperature, which exceeds the 2008 upper bound. Furthermore, the H₂/CO₂ separation mechanism of BILP-5 membranes was investigated through molecular dynamics simulations. Coupled with the merits of simple preparation, good stability, and excellent gas separation performance, BILP-5 is expected to be one of the most promising membrane materials for H₂/CO₂ separation.