Abstract: Alkali metals are important factors affecting the process of biomass pyrolysis. In this paper, rice straw (RS) with different occurrence forms of alkali metals was used as the research object. The heat decomposition characteristics, the release law of small molecule gases and the change law of in-situ pyrolysis tar composition were investigated by the thermal mass spectrometer (TG-MS) and thermal pyrolysis-GC/MS (Py-GC/MS) to reveal the action mechanism of different occurrence forms of alkali metals in the pyrolysis process. The results showed that with the increase of the removal degree of alkali metal from rice straw, the release temperature of small molecules during the pyrolysis shifted to the high temperature region, due to the catalytic effect of alkali metals on the escape of small molecules. The different occurrence forms of alkali metals had different influences on the tar components. Water-soluble alkali metals inhibited the production of alcohols and promoted the production of ketones and aldehydes. Ion-exchanged alkali metals had different effects on oil composition at different temperatures. At the pyrolysis of 300 ℃, the presence of ion-exchanged alkali metals inhibited the production of aldehydes and ethers, and promoted the production of esters and ketones, but opposite effect was obtained at the temperature higher than 400 ℃. Kinetic analysis showed that both water-soluble alkali metal ions and exchanged alkali metal could reduce the activation energy of biomass pyrolysis.