Abstract: Low smoke halogen-free flame retardant polyolefin materials have been an important material in protecting the safety of human lives and property as well as ensuring the normal production and living. However, the low flame retardant efficiency of inorganic flame retardants leads to high loading, which decreases the comprehensive performance of the materials and affect the application in high-end fields. Here, polyethylene (PE)/ethylene-vinyl acetate copolymer (EVA) composites are modified by polycarbosilane (PCS) and magnesium hydroxide (MH). The synergistic effect between PCS and MH and the effect of flame retardants distribution on flame retardancy, combustion behavior and electrical properties of composites are studied. The results show that, the combustion performance is improved with total amount of flame retardant 40%; the limiting oxygen index (LOI) of the composite reaches to 28.3; the peak heat release rate decreases by 38%; the total heat release decreases by 39%. Polycarbosilane does not affect the thermal degradation behaviors of the composites; the amorphous phase formed in the combustion process bonds the magnesium oxide particles together, forming a dense ceramic char. The two-step blending preparation method affects the distribution of MH, which is beneficial to improve the electrical properties of the composites. The volume resistivity increases by 26%, and the electrical breakdown strength increases by 22%. The research provides theoretical support and practical basis for the development and application of low smoke halogen-free flame retardant polyolefin materials in high-end fields.