The SiO₂ particles were chosen as nano-additives, and ultra-clean crosslinked polyethylene (XLPE) was used as matrix. Three kinds of SiO₂/XLPE composites were prepared with doping ratios of 1%, 2% and 3% by mass fraction. The crystallization behavior of the materials was analyzed, including the degree of crystallinity, the initial slope of the crystallization peak, the temperature difference of the crystallization peak, and the half-peak width. The microcrystalline size of the materials under X-ray diffraction testing was studied, and the changes in the breakdown property of the materials after the addition of SiO₂ particles were investigated. The results show that the addition of particles can act as heterogeneous nucleation within the ultra-clean XLPE matrix material, homogenizing the grain size distribution, but this effect diminishes with increasing particle addition. With the addition of 1% mass fraction of SiO₂ particles, the microcrystalline size of the (110) and (200) crystal planes of the material both increase to over 11%, with good grain growth, forming a stable crystalline-amorphous interface, which restricts electron migration, and the breakdown property of the material increases by 39.8%. When the doping mass fraction of SiO₂ particles is increased to 2% and 3%, the crystallization behavior of the material is restricted due to aggregation effects, resulting in a decrease in crystallinity of approximately 6% and 13%, respectively, which leads to the formation of interfacial weak zones within the matrix and an increase in free volume and the average free path of electrons, decreasing the breakdown property of the material by 6.5% and 14.3%, respectively.