To enhance the flame retardancy of high-density polyethylene/ethylene propylene diene monomer (HDPE/EPDM) crosslinked elastomers, a phosphorus-silicon synergistic flame retardant system (HDPE/EPDM/25EDAP/1VSi) was constructed using ethylenediamine phosphate (EDAP) and vinyltrimethoxysilane as flame retardants. The effects of this system on the thermal stability, flame retardancy, mechanical properties, and electrical insulation of HDPE/EPDM were investigated through thermogravimetric analysis (TGA), limiting oxygen index (LOI), vertical burning test (UL94), cone calorimetry, tensile testing, and volume resistivity measurements. The flame retardant mechanism was explored from both gas-phase and solid-phase perspectives. The results demonstrated that compared to pure HDPE/EPDM, HDPE/EPDM/25EDAP/1VSi exhibited improved thermal stability, with its LOI value increasing to 23.0% and achieving UL94 V-0 rating. The peak heat release rate (pHRR) and total heat release (THR) were reduced by 42.5% and 22.0%, respectively. Mechanistically, in the solid phase, the phosphorus-silicon structure synergistically enhanced the antioxidant capacity of the char residue and promoted char formation. In the gas phase, phosphorus-containing radicals and ammonia generated during pyrolysis effectively suppressed combustion. Regarding mechanical properties, the tensile strength and elongation at break of HDPE/EPDM/25EDAP/1VSi increased by 4.8% and 80.5%, respectively, compared to HDPE/EPDM, while the volume resistivity improved by 19.8%. This phosphorus-silicon synergistic flame retardant system demonstrates significant potential for applications in insulation and sheath materials for wires and cables.