Biomass synthesis gas production via co-reforming of biogas and H-2-rich syngas produced by biomass air-steam gasification has been investigated in a bubbling bed biomass gasifier with a NiO-MgO catalyst packed in a downstream reactor. The composition of synthesis gas was adjusted by a co-reforming reaction to meet the desired stoichiometric factor. The H-2/CO ratio of biomass synthesis gas is above 1.5. It contains trace CH4 and CO2. The feeding rate of biogas depends on the composition and flow velocity of the H-2-rich syngas produced in the gasifier. Compared with the commercial nickel-based reforming catalyst Z409R, the NiO-MgO catalyst exhibits better catalytic activity and anti-coke ability at high temperature (> 750 degrees C). Compared with conventional pathways for adjustment of the stoichiometric factor, the method of co-reforming with addition of biogas is simple and highly efficient for nearly complete utilization of the carbon contained in the biomass.