A up-down draft biomass gasification process technology was proposed,which combines the advantages of up suction and down-draft gasifier,and the corresponding fixed-bed gasifier was designed.The gasifier could creat high temperature conditions to decompose tar and completely burn carbon by means of rational distribution for gasification agent,which to achieve stable,effective and clean gasification.In addition,the thermal dynamic model of the gas composition reforming process within the secondary air region of gasifier is established.Then the model was used to perform numerical simulation for gasifier temperature distribution and outlet gas composition.