The oxygen-containing groups such as C-O, C=O, and C(=O)O were quickly introduced onto high-density polyethylene (HDPE) chains through ultraviolet irradiation in ozone atmosphere, and the functionalized HDPE was prepared. The content of the C-O, C=O, and C(=O)O groups increased with increasing the irradiation time. There was no gel in the functionalized HDPE. Compared with that of HDPE, the crystal form, cell parameters, and face space of the functionalized HDPE did not change, and its melting temperature and thermal stability decreased, while the crystallinity, hydrophilicity, and melt index increased. The functionalized HDPE/Sericite-Tridymite-Cristobalite (STC) (60/40) composite was prepared by melting blend. Compared with that of the HDPE/STC composite, the interfacial interaction and the dispersion of the STC particles in the functionalized HDPE/STC composite were improved markedly. With increasing the irradiation time, the tensile strength and notched impact strength of the functionalized HDPE/STC composite increased, while its melt index decreased.