The invention provides composite toughened autoclaved aerated concrete and a preparation method thereof. The concrete is prepared from the following main raw materials in parts by weight: 15 to 20 parts of calcined red mud powder, 24 to 32 parts of coal ash, 5 to 10 parts of gypsum, 12 to 18 parts of quick lime, 0.3 to 0.5 part of micro silicon dioxide aerogel, 2 to 3 parts of a composite toughened material, 20 to 30 parts of cement, 1 to 2 parts of a water reducer, 1 to 2 parts of polyacrylate, 2 to 5 parts of a Tris-HCI buffer solution and 10 to 18 parts of water. According to the composite reinforced and toughened concrete, halloysite nanotubes and methylcellulose which have excellent reinforcing and toughening performance are combined, and surface covalent reactions in multiple steps are conducted, so that a unique composite system is formed and can be fully dispersed in the concrete and effectively achieve the synergistic toughening effect of the materials, and therefore, the whole concrete has good tenacity, fatigue resistance and isotropy.