The invention discloses a preparation method of a high-purity mullite ceramic and a composite material. The method includes coating a SiO2 nano layer on an alumina powder surface in an ethanol solution, substituting material mixing mode by mechanical milling, drying and pre-compressing; and then sintering in a SPS sintering furnace in argon atmosphere at 1400-1500 DEG C for 5-10 min; and grinding a sintered dense sample to finally obtain the mullite ceramic block material. The preparation process of the invention has the advantages of simple operation, good repeatability, even reaction and mixing, short preparation time, low sintering temperature, uniform crystal grains and low energy cost, and can be used in the fields of refractory materials, high-temperature engineering material, electronic packaging material and optical material.