Biomass tar was a conspicuous hazardous byproduct from biomass gasification, and it has become a great challenge in the development of biomass energy technology. Here, we proposed the facile one-step synthesis of highly porous carbon using biomass tar as feedstock, porous CaO as a template, and KOH as an activation agent. Following a simultaneous activation-template-carbonization process, the product displayed tunable porosity and high specific surface area (up to 2424m(2) g(-1)). When applied to CO2 capture, the ultramicropores of this carbon material played a significant role in enhancing the CO2 affinity. In particular, the adsorbent synthesized with biomass tar: CaO: KOH in 1: 2: 3 wt ratio exhibited the high CO2 capture capacity of 5.03 and 3.13 mmol g(-1) at 273 K and 298 K (1.20 mmol g(-1) at 0.15 bar), respectively, perfect cyclic regeneration property (95% retention) and excellent initial isosteric heat of adsorption (48.55 kJ mol(-1)). The CO2/N-2 selectivity from Henry's Law and ideal solution adsorption theory reached 26.3 and 15.4 at 273 K, and 32.4 and 19.7 at 298 K, respectively.