The average molecular formula of asphaltene was calculated from the main functional groups of asphaltene determined by Fourier transform infrared spectroscopy, the average molecular weight found by gel filtration chromatography, and values of ultimate analysis. A series of average structure parameters were determined by H-1 NMR and C-13 NMR spectrum integrals, followed by establishing the structure models of asphaltene according to the Brown-Ladner method. Reduction of molecular weight of asphaltene was observed after bio-oil upgrading, and the oxygen-containing functional groups were apparently less than that of crude bio-oil while the number of aromatic rings was increased. The upgraded bio-oil had a lower molecular weight, lower oxygen content, and higher aromaticity compared with crude bio-oil.