To address the shortcomings of aluminum diethylphosphinate (ADP), such as low flame retardancy efficiency and tendency to leach under high-temperature and high-humidity conditions, this study developed a phosphorus-silicon compound flame retardant using ADP as primary flame retardant and methylphenylsilicone resin as flame retardant synergist. The aim is to create a high-performance halogen-free flame-retardant PA66 material suitable for advanced applications like new energy vehicles. The flame retardant was characterized by Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), laser particle size analysis, contact angle measurement, and water absorption tests. The results show that PA66 failed to achieve any flame retardant rating when ADP or methyl phenyl silicone resin was added alone. However, after combining the two, a phosphorus-silicon synergistic flame retardant effect was observed, and the flame retardant rating reaches UL-94 V-0. After the combination, an organic silicon layer was formed on the surface of ADP. When the methyl phenyl silicone resin content in the phosphorus-silicon flame retardant reaches 11%, the contact angle of the flame retardant increases, water absorption decreases, particle size distribution becomes narrowed, and char residue is increased. Applying the phosphorus-silicon composite flame retardant to flame-retarded reinforced polyamide 66 (PA66) can significantly improve flame retardant efficiency. When the methyl phenyl silicone resin content is 11%, the flame retardant performance is optimal: the vertical burning rating reaches V-0 at 1.6 mm, the glow-wire ignition temperature (GWIT) is increased by 25 °C, and the comparative tracking index (CTI) is increased by 225 V compared to traditional phosphorus-nitrogen systems. The tensile strength, flexural strength, and impact strength of the material remain stable. Furthermore, after 1000 h of high-temperature and high-humidity aging, surface blooming is significantly lower than that of traditional phosphorus-nitrogen flame retardant systems, and the flame retardant performance is maintained at V-0.