In order to expand the scope of C2H4 pyrolysis material and recycle the waste polyolefin, this study has employed microwave pyrolysis of heavier hydrocarbon liquids. Microwave pyrolysis not only maintains a stable energy supply to the reaction materials for obtaining a high conversion rate, but also avoids coking caused by local overheating. Steam may serve a dual role as diluent and heat carrier. Thus, this study has obtained higher olefin yield, higher olefin selectivity, and better operational continuity than conventional steam cracking. Light olefins (C-2(=)-C-4(=)) reached 50.65-68.64 mol% in the pyrolysis gas. The yields of C2H4 and C3H6 for heavier hydrocarbons pyrolysis were 42.80-44.76 and 12.98-13.91 wt% of the feedstock, respectively, while the maximum yields of C2H4 and C3H6 for pyrolysis of liquid products of polyolefin cracking were 47.00 and 22.83 wt%, respectively. Analysis of the pyrolysis mechanism indicates that the beta-scission reactions of free radicals are predominant, while the secondary reactions are significantly inhibited due to the enhanced heat transfer between the steam and the hydrocarbons.