Abstract: With the increasing waste disposal problems, high-value utilization technology using less energy is important to incentivize better recycling of plastic waste. Polyethylene high-pressure thermal cracking and catalytic pyrolysis experiments were conducted at a set temperature of 380 ℃ and low initial pressures (1−5) × 10⁵ Pa. The process temperature curves were recorded and the hydrocarbon distribution of products was analyzed. The results suggest that the phase state in the pyrolysis system is a critical issue for reaction pathways. Thus, the pressure changes have different effects on the thermal cracking and catalytic pyrolysis of polyethylene. There is a phenomenon of thermal runaway during the polyethylene high-pressure pyrolysis process. The peak temperature represents a monotonous increase with the increasing initial pressure; the higher peak temperature leads to deeper cracking of polyethylene, giving more low-molecular-weight products. In the high-pressure catalytic pyrolysis experiments under the same other conditions, no thermal runaway is observed. The Zn-supported ZSM-5 catalyst converts polyethylene into aromatics, and the selectivity for monocyclic aromatics in the liquid phase is up to 82.53%. Moreover, the coke yield is less than 1.5%.