The invention discloses a preparation method of a flame-retardant wood-plastic wallboard. The invention belongs to the technical field of wood-plastic processing, the prepared wallboard is nailable, drillable, cuttable, bondable, and can be connected and fixed by a connecting piece, and the wood-plastic wallboard has much better physical properties than that of logs, has better dimensional stability than that of wood, no cracking or warping is caused, no wood knurl or twills are generated, the wallboard can be made into various colorful products, and regular maintenance is not needed; the wood-plastic wallboard has the excellent properties of fire resistance, water resistance, corrosion resistance, moisture resistance, worm resistance, fungus resistance, acid and alkali resistance, is harmless, and is low in maintenance cost; no pollution is caused; the wood-plastic wallboard has a wood-like appearance in use, and is higher in hardness, longer in service life, better in toughness, higher in strength and energy-saving compared with log. The product has the advantages of hard quality, light weight, heat preservation, smooth and flat surface and pollution free performance.