The effect of hydrothermal pretreatment (HTP) on the biomass structure and aromatic production in the catalytic co-pyrolysis process was investigated in this study. Fundamental characteristic analysis indicated that HTP removed most of the hemicellulose and some oxygen, enhancing the hydrogen-to-carbon effective ratio (H/Ceff) of biomass. X-ray diffraction, Fourier-transform infrared spectroscopy, and solid-state cross-polarization magic angle spinning carbon-13 nuclear magnetic resonance analysis revealed that HTP mainly interrupted unstable chemical bonds (acetyl groups, glycosidic bonds, and ether linkage in lignin) and changed the biomass crystallinity. Furthermore, HTP remarkably removed alkalis and alkali earth metals (AAEM) from the biomass. To determine the effect of HTP on co-pyrolysis, individual pyrolysis of raw and pretreated corncob was carried out. Results showed that HTP significantly enhanced yields of levoglucosans and furans, both of which are important intermediates for aromatic formation during co-pyrolysis. HTP remarkably increased the aromatic contents of bio-oil in the co-pyrolysis of different corncob samples and polyethylene. The influence of HTP on aromatic production mainly included AAEM removal, hemicellulose decomposition, furan formation, and H/Ceff increments in the pretreated biomass. Thus, HTP has great potential in improving aromatic production during co pyrolysis of biomass and polyethylene.