We develop a facile strategy that converts a cost-effective porous organic polymer (denoted as PP), which is synthesized by Friedel-Crafts alkylation reaction of dichloro-p-xylene, into outstanding CO2 adsorbents through one-step post-synthesis functionalization with polyamines. Among the adsorbents synthesized, PP-2-DETA and PP-2-TEPA show superior high CO2 uptake capacities, excellent CO2 selectivity over N2, and appropriate Qst. Furthermore, the adsorbents show good CO2 uptake capacity and excellent stability toward both water vapor and regeneration cycles under dynamic flow conditions. Meanwhile, amine-functionalized organic porous polymers were used as filler materials to yield mechanically stable composite membranes that possess good CO2/CH4 separation performance. Composite membranes that contain amine-functionalized fillers exhibited both superior mechanical strength and more effective adhesion to glassy Matrimid® Polyimide (PI) and polysulfone (PSf) polymers.