Abstract: In this study, a siloxene-supported Ziegler–Natta catalyst was developed, and polyethylene/siloxene nanocomposites were successfully prepared via in-situ polymerization. Two-dimensional siloxene material was first synthesized by an etching method, and its surface was graft-modified with octadecylamine. The polar groups on the modified siloxene were then utilized to support the Ziegler–Natta catalyst. After activation with triethylaluminum, ethylene in-situ polymerization was carried out. The influence of siloxene on the catalyst properties, the effect of the supported catalyst on ethylene polymerization activity, and the variations in the properties of the resulting composites were systematically investigated. The results demonstrate that the prepared siloxene-supported Ziegler–Natta catalyst exhibits significantly higher activity than the conventional Ziegler–Natta catalyst. After polymerization, the siloxene is uniformly dispersed in the polyethylene matrix, effectively enhancing the mechanical and thermal properties of the matrix. With only 0.62% (mass fraction) siloxene addition, the tensile strength, elastic modulus, and elongation at break of the composite are increased by 129.5%, 54.1%, and 58.0%, respectively, compared with those of pure polyethylene, while the thermal decomposition temperature (θd5%) is raised by 54.3 ℃. This study provides a novel and effective strategy for preparing high-performance polyolefin nanocomposites. By overcoming the limitations of conventional materials in mechanical strength and thermal stability, it also establishes a foundation for expanding their applications in critical fields such as high-end packaging, new energy, and precision manufacturing.