The invention relates to a preparation method of an ultraviolet-irradiation-resistant heat-resistant polyethylene composite material. The method comprises the following steps: 1. covering a molecular activator on the surfaces of Al2O3 and BN nanoparticles to modify the Al2O3 and BN nanoparticles; and 2. preparing the low-density polyethylene/1 wt% Al2O3-3 wt% BN nano composite material by a blend compounding process. Nano regulation is utilized to prepare the novel ultraviolet-irradiation-resistant high-heat-resistance low-density polyethylene micro-nano composite insulation material which has engineering meanings, thereby inhibiting the injection of space charges and accumulation inside the material; and thus, the composite material has better irradiation resistance and high-temperature resistance. The composite material has wide application range in practical production engineering, has important practical and theoretical meanings in the field of engineering dielectrics, and has wider prospects.