The development process of air biomass gasification in a down-draft fixed bed gasifier was analyzed by using multi-dimensional numerical simulation method firstly and was proved to be feasible by the experimental results. Then based on the research above, the influence of the equivalence ratio RER on the temperature distribution over the entire gasifier and gas composition of the biomass gasification were investigated and predicted by numerical simulation, respectively. The results indicate that RER has significant influence on the gasification characteristics and the best range of RER is 0.24～0.28.