The invention relates to a method for preparing a lightweight mullite refractory by high-alumina fly ash and relates to the technical fields of solid waste comprehensive utilization and refractories. According to the method, waste from a power plant, namely high-alumina fly ash, is used as a raw material; commercial alumina is added according to the composition design; the mixture is fired and crushed to prepare lightweight mullite aggregate and lightweight fine powder, with different granularities; then silicon dioxide fine powder is additionally added; polystyrene spheres or sawdust is used as a pore-forming agent, aluminate cement is used as a gelling agent, and sodium hexametaphosphate is used as a water reducer; the lightweight mullite refractory is prepared through the processes such as forming by vibration pouring, curing, drying and sintering. The main high-temperature-resistant phase of the lightweight refractory is mullite phase and corundum phase; the lightweight mullite refractory comprises the main chemical constituents: 55.0-70.0 wt% of Al2O3, 25.0-40.0 wt% of SiO2 and 0-5.0 wt% of other impurities. Compared with the prior art, the method has the characteristics of wide raw material source, low production cost, low energy consumption, high mullite content, high performance, long service life, and simple operation in the preparation process; significant economic and environmental benefits can be generated.