Abstract: The online catalytic cracking of bio-oil over HZSM-5 was investigated. The HZSM-5 catalyst samples used for different reaction times were analyzed in terms of physicochemical properties and chemical compositions to investigate the effects of bio-oil catalytic online cracking. Simultaneously, the used HZSM-5 catalysts were analyzed by TG, BET, XRD, SEM and TEM to explore the mechanism of HZSM-5 catalyst deactivation. The regeneration of the deactivated catalyst was also studied. It was found that acids, aldehydes and ketones as the undesirable organics contained in the bio-oil were reduced by deoxidization over HZSM-5 and more desirable organics like phenols and aromatic hydrocarbons could be produced. Meanwhile, the activity of the catalyst declined after 80 min reaction, leading to the worse quality of the refined bio-oil. The coke deposited was mainly fibrous carbon with a little graphite carbon, where the total amount of the coke was 14.12%. The graphite coke deposited inside the pore of the catalyst and the fibrous coke deposited outside the surface of the catalyst were contributed to the deactivation of the catalyst. The basic structure of the catalyst remained unchanged, but the specific area and pore volume of the catalyst were decreased simultaneously and the agglomeration of grain was increased, leading to the decrease of crystallinity. The change of some crystal structure of the catalyst resulted in the deactivation of catalyst. After the regeneration, the catalytic activity could be recoveried.