The present study aims to improve the yields and selectivity of aromatic hydrocarbons in the catalytic pyrolysis of biomass by the addition of low-density polyethylene (LDPE), which has a higher hydrogen-carbon ratio than biomass. We have investigated the thermal decomposition behavior and kinetics, as well as the product distribution, of the co-pyrolysis of biomass (cellulose and pine sawdust) and plastic (LDPE) both with and without a catalyst (HZSM-5) using thermogravimetric analysis (TGA) and analytical pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS). Our results, based on the weight loss difference (△ W ), show that there is a positive and synergistic interaction between the biomass and LDPE. After the addition of LDPE, the synergistic reactions inhibited catalyst coking effectively and decreased the formation of solid residues. In addition, the ZSM-5 catalyst improved the reaction activity and reduced the activation energy, although the reaction mechanism is not changed. At the same time, the Flynn-Wall-Ozawa (FWO) method was used to fit the kinetic data for both non-catalytic and catalytic co-pyrolysis of biomass and LDPE, and the activation energies ( E a ) of the cellulose + LDPE + catalyst and pine + LDPE + catalyst systems were found to be 168.81 and 185.87 kJ/mol, respectively. The co-pyrolysis of biomass and LDPE effectively improved the yield and selectivity of aromatics and increased the selectivity for benzene, toluene, xylene, and ethylbenzene (BTXE). The addition of LDPE can effectively improve the selectivity for naphthalene family products (methylnaphthalene and 2-methylnaphthalene) in the catalytic pyrolysis of biomass and decrease the content of aromatic hydrocarbons larger than C10.