<p>In order to improve the utilization rate of waste resources and reduce the damage of waste plastics to the environment, the pyrolysis and catalytic pyrolysis reaction characteristics of pine sawdust (PS), high density polyethylene (HDPE) and their mixtures are analyzed. Based on the thermogravimetric (TG) analyzer and tube furnace experimental platform system, the effects of single pyrolysis, co-pyrolysis, in-situ catalysis and ex-situ catalysis on the pyrolysis behavior process and the three-phase products are investigated, and the synergistic effects of samples in the process of catalytic co-pyrolysis are clarified. The results showed that the activation energy required for the pyrolysis reaction was effectively reduced by the introduction of HZSM-5, and the pyrolysis reaction was promoted. For the co-pyrolysis of PS and HDPE, the catalyst obviously promoted the second stage pyrolysis reaction in the TG curve. Compared with in-situ catalysis, ex-situ catalysis was more beneficial to decrease tar yield and increase syngas yield, and was beneficial to the production of combustible gas. After the introduction of catalyst, the relative content of H₂ generated by the mixed sample increased from 0.147 to 0.341, and the tar yield decreased from 24.26 to 9.15%. There was carbon deposition in the pores of reacted catalyst, which can be removed by calcination to obtain a regenerated catalyst with a structure similar to that of fresh catalyst.</p>