In this work, alkali metal hydrates, barium hydroxide octahydrate (BHO) and sodium acetate trihydrate (SAT) were added into epoxy resin (EP) to prepare a series of flame-retardant EP composites. It showed that EP samples can pass the V-0 rating of UL-94 when the amount of hydrated salt exceeds 45 wt%. When the additional amount of SAT is 50 wt%, the LOI of the EP sample increases to as high as 39% compared to 19% of pure EP. In addition, the time to ignition (TTI) of EP with 50 wt% SAT was prolonged to 157 s from 63 s of pure EP, which can provide valuable time for fire escape. The peak heat release rate (PHRR) was reduced by 63.9% compared with that of pure EP (from 1392.4 kW/m2 to 502.18 kW/m2), which indicated that phase change heat absorption plays a crucial role in reducing fire hazards of EP.