Abstract: Flame retardant high density fiberboard (MPP-AP-WF/PR) composites were prepared with hot-pressing technology for wood-based products by adding melamine polyphosphate (MPP) and aluminium phosphate (AP) flame retardant to wood fiber/phenolic resin (WF/PR) composite. To investigate the optimum flame retardancy of the MPP-AP-WF/PR composites, the mass ratio of MPP to AP was explored. The effects of the mass ratio of MPP to AP on the mechanical properties, water resistance, thermal stability and flame retardancy of MPP-AP-WF/PR composites were studied based on the bending strength, thickness swelling rate, water absorption, thermogravimetic analysis and limiting oxygen index (LOI), and the flame retardant mechanism was also studied. The results show that the mechanical properties and water resistance of MPP-AP-WF/PR composites decrease significantly with the addition of flame retardant. However, the thermogravimetric results show that the flame retardant has no obvious effect on the initial heat resistance of MPP-AP-WF/PR composites, but the synergistic effect of both flame retardant at high temperature contributes to the carbon residue. LOI test results show that MPP has better flame retardant effect than AP when the flame retardant is used alone. Further, when MPP and AP are used together, the MPP-AP-WF/PR composite has the best flame retardant effect when the mass ratio of MPP to AP is 1∶2. It is due to both flame retardants exhibit the synergistic effect, which can promote the formation of dense char residue containing phosphoric acid compounds after the combustion of MPP-AP-WF/PR composite. The char can effectively prevent oxygen and heat from permeating into the internal carbon layer, thus improving the flame retardancy of the MPP-AP-WF/PR composite.