The invention discloses a microwave sintering method for preparing beta-Sialon powder by using coal ash. The microwave sintering method comprises the following steps of: adding 3-6 percent of Y2O3 sintering auxiliary agents to the beta-Sialon powder prepared from the coal ash, and ultrasonically mixing in an anhydrous alcohol medium for 1 hour to prepare slurry; then drying the slurry, and then adding 2-5 percent of polyvinyl alcohol, namely a PVA (Polyvinyl Alcohol) forming adhesive; granulating, screening, carrying out mould pressing, then carrying out isostatic pressing, and carrying out binder removal on a formed sample in a muffle furnace at 450-650 DEG C for 1-4 hours; and then heating to 1450-1600 DEG C under the condition of powder burying by utilizing microwaves, preserving heat for 15-40 minutes, and then naturally cooling to room temperature to prepare a sintered product which has good property. The microwave sintering method disclosed by the invention can be used for synthesizing the beta-Sialon powder by utilizing an industrial waste, namely the coal ash, can realize the flash sintering of the beta-Sialon powder at a low temperature within a short time in cooperation with a microwave sintering process and has the advantages of waste utilization realization, easiness in operation, low energy consumption and cost reduction.