Abstract: Based on the Gibbs energy minimization method, biomass gasification with airsteam through thermodynamics method was analyzed to get the relationship between reaction conditions and the products gas compositions. The gasification features under both atmospheric pressure and elevated pressure were computed contrastively. The correlation curve of steam to biomass ratio (S/B), which corresponds to the maximum fraction of fuel gas, and equivalence ratio (ER) under different pressures was obtained through statistic regression analysis. The computation shows that compared with gasification under atmospheric pressure, the equilibrium temperature is higher in pressurized system, and the molar fraction of fuel gas is relatively lower while methane content increases remarkably. The S/B increases with increasing pressure. More Steam is needed for pressurized gasification. By adjusting S/B, different H2/CO ratios can be obtained. The ratio of H2/CO is approximately 2 both under atmospheric and 5.0MPa pressure in the computations, in which T=1173K and S/B=1.02. The optimal S/B has a perfect linearity with ER under different pressures. As temperature equals to 1173K, the relationship of the optimal S/B and ER and pressure is expressed as following: S/B=－1.48×ER－4.49 E×10－5×p2 + 5.83 E×10－3×p + 0.32.