The hydrogen-deficient and oxygen-rich nature of lignocellulosic biomass prohibits effective conversions of biomass to fuels and chemicals via catalytic pyrolysis due to significant coking of the catalysts. Co-feeding of biomass feedstock with hydrogen-rich and oxygen-deficient thermoplastics could improve the process. Herein, thermal and catalytic co-pyrolysis of cellulose and polyethylene (PE) was studied via thermogravimetry combined with an online photoionization time-of-flight mass spectrometry (PI-TOF-MS). No notable synergetic effect was found in the thermal co-pyrolysis process while a considerable synergetic effect was observed during the catalytic co-pyrolysis. In the case of catalytic pyrolysis, co-feeding of cellulose with PE significantly improved the aromatic formation. Detailed reaction intermediates and products were detected by PI-TOF-MS and the process of aromatization could be ascribed to aromatization of small oxygenates and olefins, as well as Diels-Alder reaction and dehydration by HZSM-5. Moreover, this study provides a reliable tool for screening and optimizing of catalytic co-pyrolysis.