Abstract: The invention discloses a high impermeability and low thermal conductivity inorganic lightweight foam concrete and its preparation method, including the following mass fraction of raw materials: 1260˜1540 parts of ordinary silicate cement, 20˜60 parts of nano-silica, 460˜740 parts of fly ash, 360˜440 parts of aggregate, 9˜11 parts of redispersible latex powder, 7.2˜8.8 parts of polypropylene fiber, 27˜33 parts of quick-setting agent, 500 parts of fluorine-free foam, 900˜1100 parts of water. 33 parts, 500 parts of fluorine-free foam, 900˜1100 parts of water. The high impermeability and low thermal conductivity inorganic lightweight foam concrete prepared by the present invention has a simple formulation, good workability, light weight and low thermal conductivity, and is suitable for the construction of thermal insulation system for building exterior walls.