The invention relates to a preparing method of porous mullite ceramic. The ceramic is prepared from bauxite and diatomite with the steps of wet grinding, filtering, stoving and stirring by adding water. And binder, dispersant, foam stabilizer and frother are added, then forming, stoving and sintering are conducted to prepare the porous mullite ceramic. According to the preparing method of porous mullite ceramic, the raw material is easy to obtain, preparing technology is relatively simple, large special equipment is not needed, investment is less, production cost is relatively low, product production cycle is short, efficiency, purity and porosity are high, product quality is stable, the porous mullite ceramic can be prepared as required, porosity rate of the porous mullite ceramic is 88-93%, and the porous mullite ceramic has relatively good industrial utilization prospect.