Abstract This study aimed to understand the mechanism of dual catalytic effects of inherent alkali and alkaline earth metallic species (AAEMs) on biomass gasification. Two kinds of typical Chinese agricultural biomass were gasified using updraft quartz reactor with steam. The results indicated that external steam had negligible effects on promoting further thermal cracking or reforming of tar under 900 °C. The presence of AAEMs enhanced the production of H2 and CO2, while inhibited the production of CO、CH4、C2H4 and C2H6. The heterogeneous char-steam reaction, as well as the homogeneous hydrocarbons reforming and water-gas shift reactions were promoted by the presence of AAEMs. Alkaline earth metals had more significant catalytic effects on water-gas shift reaction compared to alkali metals. The results from UV fluorescence spectra further proved that the additional steam had negligible promoting effects on secondary reforming of tar, while the inherent AAEMs had a significant catalytic role in thermal cracking and reforming of tars.