Abstract: In recent years, plant fiber reinforced composites have been widely concerned because of their advantages of environmental protection, high specific modulus and low cost. In order to study the water absorption characteristics of flame retardant treated plant fiber reinforced composites, flax fiber reinforced phenolic composites (Flax/Phenolic, FP) were used as the research object. The flame retardant treatment of flax fiber was carried out by grafting dimethyl methylphosphonate (DMMP), and the optimal treatment process was explored. The water absorption of DMMP grafted composites and its influence on the flame retardancy and tensile properties of the composites were studied, compared with the physical impregnation method. The results show that DMMP grafting method has higher flame retardant efficiency and better water absorption resistance. Compared with physical impregnation method, the chemical bond formed by DMMP grafting is difficult to be destroyed by water molecules. After 30 days of water absorption, the limiting oxygen index (LOI), vertical combustion, heat release and smoke release of DMMP grafted composites have not changed significantly compared with those before water absorption, and the flame retardancy of DMMP grafted composites is well maintained after water absorption. On the other hand, the negative effect of grafting on the tensile properties of composites after water absorption is also lower than that of physical impregnation.