Biomass energy has drawn increased attention owing to its zero carbon emissions and large reserves. Pyrolysis instead of direct combustion is an efficient and clean way for biomass energy conversion. As a strong radiative species, H2O greatly affects the combustion processes. Despite the large concentration and fluctuation of H2O content in biomass pyrolysis gases (py-gas), radiation reabsorption effect has not been systematically investigated for py-gas combustion. In this study, one-dimensional simulations of premixed py-gas/air flames were conducted, using PREMIX code with both adiabatic and radiative models. The effects of H2O content and radiation reabsorption were examined. As H2O content in the py-gas mixture increased from 40% to 50%, the flame speeds decreased from 42.86 to 28.08 cm/s, while the relative differences caused by radiation reabsorption increased from 9.92% to 17.42%. Radiation reabsorption affected laminar flame speed primarily through the preheat-induced chemical effect, which was mainly controlled by HCO radical. The outlet mole fraction of NO was reduced by up to 13.56% when radiation reabsorption was considered. Reaction pathway analyses revealed that the NO emission was closely related to the outlet temperature and the peak NCO concentration, which were the limiting factors for the thermal-NO route and the fuel-NO route, respectively.