The invention discloses a mullite fiber preparation method which is efficient, rapid, environment-friendly and low in cost. The method includes: taking raw materials according to stoichiometric ratio of elements in the mullite fiber chemical formula, preparing colloid, performing fiber forming and drying for colloid discharging to obtain mullite non-crystal fiber, adding the fiber into a sagger made of wave permeable materials, placing the sagger in a special microwave oven, controlling heating rate by adjusting the microwave power under the air or oxygen atmosphere, heating to a heat treatment temperature, and cooling to the room temperature to obtain the mullite fiber. The mullite fiber prepared by the method is high in impurity and low in porosity, the grain size is about 30nm, and the fiber diameter ranges from 6 micrometers to 10 micrometers. Further, the mullite fiber is excellent in uniformity, brittle fracture and chalking of the fiber are greatly reduced, yield is increased, heat treatment temperature is lowered, heat insulation time is shortened, and production efficiency is greatly improved.