Biomass pyrolysis is a promising method and often used to reduce hexavalent chromium Cr(VI) residue. However, the pyrolysis reduction mechanisms of Cr(VI) remain unclear, particularly regarding which tar compositions help the Cr(VI) reduction. In the present study, on-line MS analysis suggests that yields of CO and CH4 from biomass pyrolysis can principally represent the rate of Cr(VI) reduction. FT-ICR MS results indicate that high-molecular-weight compounds in tar (m/z > 350) located in the lignin/CRAM-like structure region or saturated (H/C=0.7-1.5) and oxidized (oxygen number>6) compounds preferentially crack by catalysis of Cr (VI) to produce CO via decarbonylation reaction, further promoting the reduction of Cr(VI). Pyrolysis material with low stability (low aromaticity) can release more reducing gases and more easily cracks tar to promote Cr (VI) reduction. Lastly, chromite slag from a polluted site was successfully remediated by pyrolysis of a hydrochar sample. An extremely low leaching Cr concentration from remediated chromium slag can be obtained due to the carbon layer coating on Cr element. This study provides a novel perspective on the pyrolysis remediation mechanism of Cr(VI) and is highly significant for developing thermal remediation techniques for polluted sites.