Abstract: The influence of biomass tar on the overall thermal chemical process has aroused much more attention with the development of biomass gasification technology. Catalytic cracking is thought to be the most effective route for the solution of biomass tar elimination. However, the variation of tar constitution during the catalytic cracking process, which is most valuable for understanding of the process mechanism, is poorly known. One kind of chromatography was used to analyze the constitution of biomass pyrolysis tar, which results in good separation effect and high accuracy. The constitution of tar from different biomass sources and different catalytic cracking conditions was compared, and also the mechanism of catalytic cracking was discussed. On account of the composition of different biomass feedstocks, the content of aromatic and polar fraction in the tar from wood chip pyrolysis is higher than those from rice straw and rice husk. With the increase of pyrolysis temperature, the content of aromatic fraction in tar also increases. After catalytic cracking, the content of aromatic fraction in tar almost doubles, and is 50% with the cracking temperature above 950 ℃. However, other fractions decrease to different extents. The reason lies in that the reaction rate of aromatic fraction is smaller than those of other fractions, and also there is some conversion from other fractions to aromatic fraction. During catalytic cracking, the constitution of biomass tar has the trend of aromatization, which becomes obvious with the extent of tar conversion.