Abstract: To guide and modify the design and operation of small biomass gasifier, the design procedure of an updraft fixed bed gasifier was introduced in detail, and its structure was optimized, then a biomass gasifier was produced. Using sawdust as raw material, the influence of air flow rate on gasification temperature, product gas composition and low heating value (LHV) was investigated. The results showed that: air flow rate was the key parameter during the actual operation process of the gasifier, it directly affected the gasification temperature and operation condition. As air flow rate increasing, the temperature inside the gasifier rose greatly, the gas production increased sharply, and the combustible gas content first increased and then decreased. The optimum air flow rate was about 1.9m3/h, where the LHV of the product gas got the maximum value of 4.38 MJ/m3, the oxidation area had a suitable temperature of 960℃, and the gasification intensity was 57.8 kg/(m2·h), it can well meet the requirements of user.