In this paper, nanoporous nickel phosphide (Ni12P5) was synthesized by using a novel hydrothermal method, and its structure was characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Then it was used as a synergistic agent with intumescent flame retardant (IFR) in the ethylene-vinyl acetate (EVA) copolymer. With the addition of 2 wt % Ni12P5 and 28 wt % IFR, the LOT value increased from 30.5 to 34.5. The UL-94 test showed that EVA with 25 wt % IFR burned and had no rating, but with the addition of 2 wt % Ni12P5 and 23 wt % IFR, it could reach V-0 rating. The CCT results revealed that the Ni12P5 and IFR system could result in excellent flame retardance. The TGA data indicate that Ni12P5 can increase the thermal degradation temperature and the charred residues after burning. Moreover, the mechanical and electrical properties of EVA composites are also investigated.