Fresh HZSM-5 catalyst was hydrothermally treated with 20% steam partial pressure at 450, 500, and 550 degrees C. Catalytic upgrading of corn stalk fast pyrolysis vapors with fresh and hydrothermally treated HZSM-5 catalysts was studied by means of analytical Py-GC/MS. Hydrothermal treatment caused a reduction in the surface area, micropore surface area, pore volume, micropore volume, pore size, total acid sites (both weak acid sites and strong acid sites), and coke yield of the HZSM-5 catalyst. The product distribution and the quality of the pyrolysis vapors were also affected. HZSM-5 catalyst with higher hydrothermal treatment temperature resulted in a higher relative content of hydrocarbons and lower relative contents of acids, esters, sugars, and furans. Esters, sugars, and furans were completely converted when HZSM-5 catalyst with 550 degrees C hydrothermal treatment was used. The relative contents of alcohols, carbonyls, and phenols first increased and then decreased sharply with a rise of hydrothermal treatment temperature. The increase of hydrothermal treatment temperature was also conducive to the decrease of oxygen content in the organic pyrolysis vapors.