Membrane technology is attracting more and more attention in hydrogen energy and natural gas sweetening applications. One challenge remaining is advanced membranes with both high separation performance and robust. Herein, remarkable gas separation properties of carbon molecular sieve membranes (CMSMs) are obtained by debromination assisted carbonization for a series of brominated intrinsic microporous polyimides (PIM-PIs). The di-bromine substituted 6FDBPI shows over 10 times higher permeability of CO₂ (2147 vs 184 Barrer) and similar CO₂/N₂ selectivity (19.7 vs 22.7) than the non-bromine substituted 6FNBPI. This is due to the bromine introduction creates a large number of ultra-micropore channels. After carbonization and debromination at 550 °C, the CMSM-550s exhibit a smaller (4.20–4.62 vs 4.77–8.49 Å), more intensive (75–90% vs 5–35%) and higher percentage (∼80% vs 10–55%) of ultra-micropores than their pristine PIM-PIs. Consequently, the CMSM-550s show both higher permeability and selectivity than their precursors, the larger permeability originates from the higher diffusion and solubility coefficient, the higher selectivity is purely contributed by diffusion selectivity. Notably, the 6FDBPI-550 shows an unprecedented H₂ permeability of 30,943 Barrer and H₂/N₂, H₂/CH₄ selectivity of 47.5 and 46.6 that by far exceed the latest trade-off lines, coupled with excellent anti-plasticization and mixed-gas separation properties. This high performance together with the bromine facilitate carbonization method provide great potential in H₂ enrichment and natural gas sweetening applications.