In this work, an aqueous pentaethylenehexamine (PEHA) solution was studied for CO₂ removal from bio-syngas for the first time. Firstly, pure CO₂ absorption in aqueous PEHA solution under different conditions was conducted, and 20 wt% PEHA solution was identified as the best option. Secondly, the capture of CO₂ was tested with synthetic syngas from a gas cylinder, and the species other than CO₂ showed a negligible impact on CO₂ removal. Finally, to evaluate the practical feasibility of using aqueous PEHA solution on the downstream CO₂ capture, the pilot experiments of gasification with boreal forest-based biomasses were designed to provide real syngas with a realistic distribution in composition for further testing. The results showed that the operating conditions and the type of feedstocks affected the distribution in the bio-syngas composition. Among these feedstocks, at the optimal oxygen supply, using spruce needles generated the highest yields of CO and H₂ and, meanwhile, gave rise to similar yields of other gases such as CO₂, CH₄, etc. The influence of the species other than CO₂ for CO₂ removal was negligible. Additionally, aqueous PEHA solution was tested as a biomass pretreatment agent, showing that no significant changes could be identified by the ultimate analysis (except for increased nitrogen content), but the yields of CO were affected negatively. On the other hand, when using the pretreated biomass by the aqueous PEHA solution, the NH₃ concentration in bio-syngas reached to the highest (4000 parts per million), which slightly affected the CO₂ absorption capacity and initial absorption rate of 20 wt% PEHA solution in a positive way.