An energy-saving high-temperature light-weight multiphase brick belongs to the technical field of novel bricks and comprises a brick body, a hard layer is arranged in the middle of the interior of the brick body, sparse layers are arranged on the upper side and the lower side of the hard layer, and polishing layers are formed on the outermost layers of all parts of the brick body through polishing and grinding. The hard layer, the sparse layer and the polishing layer are all formed by mixing and stirring mullite particles, clay and other alkali-resistant materials according to a certain proportion, molding the mixture through a mold and then firing the mixture, and a large number of stable micropore structures exist in the structure formed by firing the mullite particles, the clay and the other alkali-resistant materials. An alignment groove B is formed in the middle of the top of the brick body in the length direction, an alignment groove A is formed in the top of the brick body in the width direction, a through hole penetrating through the brick body is formed in the middle in the brick body in the length direction, and heat dissipation grooves are fixedly formed in the middles of the two long side faces of the brick body in a concave mode. The mullite-alkali-resistant composite brick is made of light mullite and alkali-resistant materials in a complex phase mode, a stable micro-pore structure is formed in the brick, and the brick has the advantages of being high in strength, high in alkali corrosion resistance, low in heat conductivity, stable in high-temperature performance and the like.