To develop composite separator materials with excellent flame retardancy, electrical properties, and mechanical properties to meet the safety requirements of power battery systems for new energy vehicles, we prepared a series of ATO/PS composites by adding different mass fractions of nano antimony tin oxide (ATO) to polystyrene (PS). The micromorphology, mechanical properties, flame retardancy, electrical properties, and corrosion resistance of PS and ATO/PS composites were systematically compared. The results show that when the ATO mass fraction is 4%, the ATO/PS composite achieves the best comprehensive performance. Compared with pure PS, its tensile strength, elongation at break, flexural strength, and impact strength increase by 27.75%, 36.71%, 20.31%, and 26.54%, respectively; the flame retardancy is significantly improved, with the limiting oxygen index increasing by 23.7% and the UL-94 rating upgrading from NR to V-0; in terms of electrical properties, the volume resistivity decreases by 77.5% and the dielectric loss factor decreases by 51.5%. When the ATO mass fraction is 5%, excessive ATO leads to a decline in mechanical properties due to the agglomeration effect, and the improvements in flame retardancy and electrical properties slow down.