[en] This study investigated the synergy in the co-pyrolysis of cellulose and polyethylene (PE) over HZSM-5 and the mechanism of the catalytic co-pyrolysis. PE, cellulose, cellulose/PE, cellulose/PE/HZSM-5 were subjected to thermogravimetric analysis and Py-GC/MS experiments at 650 °C to obtain the distribution of products, respectively. The results of Py-GC/MS experiments showed that the products quality of co-pyrolysis of PE and cellulose is improved over HZSM-5. The mechanism of catalytic co-pyrolysis was mainly the radical reaction and Diels–Alder reaction. The catalytic effect of HZSM-5 was mainly to promote the Diels–Alder reaction to produce more aromatic hydrocarbons. According to the density functional theory, the mechanism of this catalytic co-pyrolysis was verified, and the proposed reaction paths were verified to make the co-pyrolysis process more clear.