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The accumulation of waste plastics has been one of the most urgent and difficult environmental challenges. In this work, the pyrolysis of polyethylene (PE) has been studied by thermogravimetric analysis (TGA), pyrolysis/gas chromatography/mass spectrometry (PyGC-MS), and a two-stage tube reactor. The pyrolysis behavior of high-density polyethylene (HDPE) and low-density polyethylene (LDPE) is compared, and there are almost no differences in the thermogravimetric curves obtained by TGA and the yield of gaseous products obtained in a two-stage process. The evolution of light olefins (C 2 –C 4) during the pyrolysis of PE is investigated in a two-stage process with a constant condition in the first stage (500 °C) and various conditions in the second stage (700–1000 °C, 0.2–0.4 s). The temperature in the second stage has a more significant effect on the yield and selectivity of light olefins than the residence time, and the ethylene, propylene, and butene yields do not increase monotonously as the temperature rises. The highest yield of light olefins of 76.1 wt% is obtained at 800 °C with a selectivity of 91.7%. The maximum yields of ethylene, propylene, and butene are 45.1 wt%, 17.3 wt%, and 24.3 wt%, respectively. The scission mechanisms are proposed to provide insight into the process during which PE molecules transform into light olefins. • There are almost no differences between the pyrolysis characteristics of high-density and low-density polyethylene. • The evolution of light olefins during the polyethylene pyrolysis is investigated by PyGC-MS and a two-stage tube reactor. • A high yield of light olefins of 76.1 wt% with a high selectivity of 91.7% is obtained at 800 °C. • Probable mechanisms for two-stage pyrolysis of PE are proposed to reveal the interplay of reactions. [ABSTRACT FROM AUTHOR]