Two layers of air distribution were designed to actively control the agent feed in down-draft fixed bed biomass gasifier. The pressure and velocity distribution inside the gasifier were studied by simulation analysis. The pressure field displayed the law of step distribution and obvious boundaries. The velocity pattern was higher from top to down with high level of uniformity in the same section. Experiments were done to test the pressure and temperature distribution in the gasifier by using corn stalk as materials. The results showed that it had an important effect of active air distribution on promoting the reaction uniformity. The temperature kept stability at the same section with little changes of the gas components, and calorific value kept above 5 500kJ/m3.This method can provide a new way to improve the working performance of the fixed gasifier.