The HDPE/nano-SiO₂ composites were prepared using silane coupling agent (KH-570) to modify nano-SiO₂ and mix nano-SiO₂ with high-density polyethylene (HDPE). The mechanical properties and heat resistance of HDPE/nano-SiO₂ composites were explored. The results showed that KH-570 was successfully grafted onto the surface of nano-SiO₂. With the increase of nano-SiO₂ content, the tensile strength, flexural strength and impact strength of HDPE/nano-SiO₂ composites first increased and then decreased. When the nano-SiO₂ content is 3%, the mechanical properties and heat resistance of HDPE/3%nano-SiO₂ composites reach the best, and their tensile strength, bending strength and impact strength reach 39.6 MPa, 32.5 MPa and 16.9 kJ/m², respectively, which are 73.7%, 59.3% and 36.3% higher than those of pure HDPE, respectively, and the initial thermal decomposition temperature of HDPE/3%nano-SiO₂ composites is increased by 101 ℃ compared with pure HDPE, the quality retention rate of HDPE/3%nano-SiO₂ is 6.2%.