To improve the flame-retardant properties of ethylene-vinyl ecetate copolymer (EVA) with aluminum hydroxide (ATH), a blend of aluminum diethylphosphite (ADP) and ATH was used. The flame-retardant properties of the composites were evaluated by using vertical burning (UL-94), limiting oxygen index (LOI), and cone calorimeter (CONE) tests. The results indicated that at the mass ratio of ATH to ADP of 2∶1 and 1∶2, compared to the EVA/ATH composites, the vertical burning rating improved from no rating to V-0, and the LOI increased from 34.5% to 37.8% and 42.8%, respectively. Different methods were used to analyze the flame-retardant mechanisms of samples with these two ratios. For the sample with a mass ratio of ATH to ADP of 2∶1, the content of phosphorus-containing compounds released during thermal decomposition was extremely low, and the formation of aluminum phosphate (AlPO₄) strengthened the char layer, made it more continuous and dense, resulting in higher residue mass than theoretical value. Therefore, the proposed flame-retardant mechanism at this ratio is dominated by the condensed phase. For the sample with a mass ratio of ATH to ADP of 1∶2, the char layer had more pores, and the residue mass was lower than theoretical value, but high content phosphorus-containing compounds appeared in the gaseous products. Hence, the proposed flame-retardant mechanism at this ratio is dominated by gas phase.