Polyethylene-based waste plastics need hundreds of years to degrade in atmospheric conditions, so innovative upcycling processes are necessary in addition to traditional recycling services. This study presents an environmentally benign and solvent-free autogenic process, in which waste plastics such as waste polyethylene (PE) were converted into Fe3O4@C core-shell structures with about 800 nm in diameter and tens of micrometers in length with the presence of catalysts while oxygen was absent. The composition and morphology of the as-obtained Fe3O4@C core-shell structures were characterized by advanced structural, spectroscopic, and imaging techniques. The magnetic measurement at room temperature indicates that the values of saturation magnetization (22.5 emu/g) and coercivity (152.9 Oe) of the one-dimensional Fe3O4@C core-shell structures are different from those of Fe3O4 nanoparticles and bulk Fe3O4 due to the different carbon content, dipolar interaction, size, and morphology of the products. The results indicate that the one-dimensional Fe3O4@C core/shell structures possess well acid resistance.