Active air distribution way was taken to the experimentally study on the tar cracking property in downdraft fixed-bed biomass gasifier. The results showed that the reaction was uniform in the same section and the gasifier had a thicker layer at high temperature under the active air distribution, which provided good conditions for tar cracking. The optimum air distribution way was achieved by the experiments, and the tar content in gas was about 600 mg/m3 in the range using corn stalk as the material with high heat value about 5400 kJ/m3. The effect of ash layer thickness and the other factors on tar cracking and gas quality was also studied. 