Abstract: To address the issues that high-density polyethylene (HDPE) exhibits poor flame retardancy and traditional flame-retardant modification methods tend to impair its mechanical and electrical properties, a halogen-free block copolymer (PAPO-MP) was synthesized using piperazine pyrophosphate (PAPP) and melamine polyphosphate (MPP) as raw materials. This copolymer was then used as a flame retardant to prepare PAPO-MP/HDPE composites. Combined with experimental tests and molecular dynamics (MD) simulations, the effects of PAPO-MP on the flame-retardant, mechanical, and electrical properties of HDPE were systematically analyzed. The experimental results show that when the mass ratio of PAPP to MPP is 7∶3, the PAPO-MP/HDPE composite achieves the optimal flame retardancy: the limiting oxygen index (LOI) is increased to 30.2%, and the flame-retardant grade reaches UL-94 V-0. Meanwhile, under this ratio, the composite exhibits excellent mechanical and electrical properties, with a tensile strength of 16.2 MPa, an elongation at break of 224.5%, a dielectric constant of 2.093, and a dielectric loss factor as low as 2.2%. Furthermore, MD simulations were performed to calculate the mechanical and electrical property parameters of the PAPO-MP/HDPE composite. The simulation results indicate that the composite also achieves the best mechanical and electrical properties when the mass ratio of PAPP to MPP is 7∶3, which is consistent with the experimental results. The PAPO-MP/HDPE composite designed and prepared in this study possesses outstanding flame retardancy, mechanical strength, and electrical properties, providing experimental and theoretical basis for the optimization of the comprehensive performance of HDPE.