Abstract: Nano sepiolite (hereinafter referred to as sepiolite) was used as a synergist to prepare flame-retardant ethylene-vinyl acetate copolymer (EVA) composite materials. Influence of sepiolite on the mechanical, thermal, and flame retardant properties of magnesium phosphorus flame retardant EVA was studied through hardness, tensile testing, thermogravimetric analysis (TGA), oxygen index (LOI), and cone calorimeter testing. Results showed that with the increase of sepiolite addition, hardness of samples showed an upward trend, breaking strength showed a downward trend, percentage of breaking elongation showed a trend of first increasing and then decreasing. In terms of insulation resistance, when the addition of sepiolite was less than 2 g, the volume resistivity of EVA composite materials increased. when the addition of sepiolite was more than 2 g, the volume resistivity of EVA composite materials decreased sharply. In terms of thermal properties, with the increase of sepiolite addition, the initial melting temperature, melting peak, crystallization temperature, and crystallinity of EVA composite materials increased first and then decreased. There were three stages of weight loss during heating process, the first decomposition peak temperature changed slightly when the addition of sepiolite was less than 2 g, and decreased significantly when the addition of sepiolite was more than 2 g. The second decomposition peak temperature decreased from 383.8 ℃ to 375.9 ℃ with the increase of sepiolite addition, and the third decomposition peak temperature remained basically unchanged. In the limiting oxygen index (LOI) test, LOI gradually increased with the increase of sepiolite addition, rising from 32.7% to 34.7%. In the cone calorimeter test, adding an appropriate amount of sepiolite could effectively reduce fire behavior parameters, such as heat release rate (HRR), total heat (THR), total smoke release (TSP), and mass loss rate (MLR). The ashes photos showed that a dense crust layer with flame retardants could form when the addition of sepiolite added was 2 g, which could hinder the spread of flames inward.