Producer gas from biomass gasification is a feasible supplementary source to overcome the shortage of energy supply. However, the relatively high CO content and low heating value greatly restrict raw biomass producer gas for extensive application. The technical combination of shift-methanation reaction and CO2 adsorption is proved to be an effective upgrading method. Different from the conventional tandem way of connecting shift-methanation and CO2 adsorption, this work first attempts to integrate the two units into a fixed-bed reactor to form a cooperative symbiotic relationship. The relay upgrading process of biomass producer gas is numerically investigated over Ni-based catalyst and mesoporous carbon adsorbent using computational fluid dynamics method. It is found that the CO content can be effectively reduced from 30.8% to below 5% and lower heating value on wet basis can be increased from 6.5 MJ/Nm3 to over 10 MJ/Nm3. The influence of catalyst and adsorbent bed volume on the composition of product gas is evaluated. Meanwhile, the effect of catalyst and adsorbent bed gap is also analyzed. Given the similarity of catalyst and adsorbent bed, the impact of their sequence on the upgrading performance is discussed. At last, for continuous run, a regeneration cycle involving reaction–adsorption coupled with N2 purge is designed and the cycle simulation is conducted to better understand the flow behavior and reaction/adsorption characteristic.