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 the primary flame retardant and methylphenylsilicone resin as a 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 (FTIR), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), laser particle size analysis, contact angle measurement, and water absorption tests. The results showed 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 reached UL-94 V-0. After the combination, an organic silicon layer formed on the surface of ADP. When the methyl phenyl silicone resin content in the phosphorus-silicon flame retardant reached 11%, the contact angle of the flame retardant increased, water absorption decreased, particle size distribution narrowed, and char residue increased. Applying the phosphorus-silicon composite flame retardant to flame-retarded reinforced polyamide 66 (PA66) significantly improved flame retardant efficiency. When the methyl phenyl silicone resin content was 11%, the flame retardant performance was optimal: the vertical burning rating reached V-0 at 1.6 mm, the glow-wire ignition temperature (GWIT) increased by 25°C, and the comparative tracking index (CTI) increased by 225 V compared to traditional phosphorus-nitrogen systems. The tensile strength, flexural strength, and impact strength of the material remained stable. Furthermore, after 1000 hours of high-temperature and high-humidity aging, surface blooming was significantly lower than that of traditional phosphorus-nitrogen flame retardant systems, and the flame retardant performance was maintained at V-0.