Abstract: A reactive halogen-free flame retardant composite (OP-FRC) was prepared by compounding unsaturated polyester oligomer (UPEO) with inorganic flame retardants, and then the composite was introduced into ethylene-vinyl acetate copolymer (EVA)-based low-smoke halogen-free flame retardant cable sheath material via melt blending. Influence of UPEO on the structure and properties of the material was investigated using a thermogravimetric analyzer, differential scanning calorimeter, parallel plate rheometer, electronic universal testing machine, scanning electron microscope, and limit oxygen index tester. The results indicated that the incorporation of UPEO in OP-FRC improved the dispersion of inorganic flame retardants in the EVA matrix, enhanced the interaction between them, facilitated the formation of an inorganic filler network, increased the mobility of molecular segments, promoted the relaxation of the entangled matrix structure, and thus improved the fatigue resistance, thermal stability, and crystallization ability of the materials. When the mass fraction of UPEO was 4%, the material exhibited an elongation at break of 215.5%, a fatigue life of 26354 cycles, and a limiting oxygen index (LOI) of 31.5%.