The study aims to prepare modified glass fiber (mGF) through KH550 modified glass fiber (GF), aiming to enhance the mechanical and insulation properties of PA66 composites, and explore the effects of mGF content on the mechanical and insulation properties of PA66 composites. The results show that as the mGF content increases, the density of the composites shows a trend of first increasing and then decreasing. The tensile strength, elastic modulus, and notch impact strength all show a trend of first increasing and then decreasing. When the mGF content is 20%, the enhancement effect of various mechanical properties of PA66 composites is the strongest, with an increase of 13.38%, 19.23%, and 19.59% compared to the tensile strength, elastic modulus, and notch impact strength of pure PA66 composites, respectively. The electric field breakdown strength of PA66 composites shows a trend of first increasing and then decreasing under both DC and AC conditions. At the same time, the dielectric constant of PA66 composites shows a trend of first decreasing and then increasing, and reaches its lowest point at a dosage of 30%. The volume resistivity of PA66 composites shows a trend of first increasing and then decreasing, and reaches its highest value at a dosage of 20%, which is 3.34×10¹³ Ω·m. The mechanical and insulation properties of PA66 composites can be enhanced by adding 20% KH550 modified GF, which can be applied in the preparation of electrical circuit breaker materials. This material can be used as an electrical circuit breaker material.