The invention discloses a preparing method of flame retardant plywood. The method comprises the steps of log machining, primary drying, immersion treatment, secondary drying, gluing, pre-pressing, hotpressing and edge cutting and sanding; according to immersion treatment, a fire retardant is prepared into an immersion solution, the solution concentration ranges from 5 to 10%, a single board obtained after primary drying is immersed in the immersion solution to be immersed, the immersion time ranges from 3 hours to 6 hours, and the fire retardant comprises, by weight part, 30 to 40 parts of phosphoric acid, 10 to 20 parts of ammonium dihydrogen phosphate, 8 to 15 parts of squinamide phosphate, 10 to 20 parts of melamine phosphate, 10 to 20 parts of aluminum dihydrogen phosphate, 3 to 5 parts of phosphorus-contained polysiloxane and 2 to 4 parts of methyl phenyl silicone resin. When the fire retardant is heated, phosphorus and silicon synergistic effect is generated, the stable carbon layer is formed in the system, the residual carbon amount reaches above 60%, heat stability is good, heat isolation, oxygen isolation and smoke restraining are generated, and the molten drop preventionaim is achieved.