Expanding the application field of polyolefin materials through functionalization has been a research hotspot in the past three decades. Here, a TiO2-supported anilinenaphthoquinone nickel catalyst was assembled and applied for in situ ethylene polymerization with high activity (>2000 kg mol–1h−1) to produce ultra-high molecular weight polyethylene (UHMWPE)/TiO2 composites with unique physicochemical performance. The UHMWPE/TiO2 composite films and fibers prepared by in-situ ethylene polymerization are superior to the samples from the blend system in issues such as TiO2 dispersibility, mechanical property, and photocatalytic degradability. The mechanical properties (strength up to 26.8 cN/dtex, modulus up to 1248.8 cN/dtex) of the obtained UHMWPE/TiO2 composite fibers are significantly improved with a very low dosage of TiO2 (as low as 1.4 wt‰). Moreover, UHMWPE/TiO2 composites obtained by coating Al2O3 and SiO2 on the surface of TiO2 not only retain the strong absorption of ultraviolet rays, but also effectively weaken the photocatalytic degradation effect.