A series of covalent triazine frameworks containing benzimidazole segments (CTF-BIs) were designed and prepared by dynamic trimerization under ionothermal conditions, based on the novel designed dicyano monomer of dicyano benzimidazole (DCBI). The CTF-BIs were all amorphous and exhibited BET surface areas up to 1549 m² g⁻¹. The effect of reaction conditions including the ratio of catalyst and temperature on the pore properties was discussed. Meanwhile, the CO₂ adsorption capacity of all the CTF-BIs was studied and it was found that the CO₂ uptake capacity was as high as 21.68 wt% at 273 K and 1.10 bar for CTF-BI-11. The CTF-BIs exhibited high selectivities for CO₂ over N₂ in the range of 31.3–88.5 at 273 K, and 29.0–102.7 at 303 K, respectively.