Porous ceramics are-widely used in a variety of environmental fields such as solid–liquid separation and hot gas filtration due to their high mechanical strength, excellent chemical resistance, and high thermal stability. Conventional porous ceramics are usually made of particle-packing of inorganic powders, which are usually characterized by a low porosity, a high self-weight, a low permeability and a high filtration resistance. It is thus necessary to develop novel porous ceramics with a high porosity, a high mechanical strength and a low packing density. Mullite whisker-based porous ceramics have attracted recent attention in the treatment of various wastewaters. Compared with porous ceramics prepared by conventional particle-packing, mullite whisker-based porous ceramics are constructed by whisker interlocking, resulting in fibrous structures with a high porosity, a high pore connectivity and a low packing density. However, these mullite porous ceramics are seldom used in industry due to the high cost of the high-purity starting materials. Fly ash is a major by-product of coal combustion. The elemental analysis shows that fly ash is mainly composed of unburned carbon, metal oxides (i.e., Si, Al, and Fe) and other inorganic materials. Fly ash can be used to prepare mullite porous ceramics due to the presence the high content of aluminum silicates. However, the effect of holding time on the growth of mullite whiskers and the permeance-selectivity of the fibrous structures is still unclear. In order to elucidate the growth mechanism of mullite whiskers based on gas-solid reaction, the effects of sintering temperature and holding time on the microstructure, phase composition and filtration properties of the porous ceramics were investigated.