Calcined flint clay has a large reserve in China. In this paper, porous mullite ceramics are prepared by calcined flint clay, while the sawdust is pretreated with different concentrations of silica sol. Effects of sintering temperature and silica sol concentrations on physical properties and microstructures of the porous mullite ceramics are studied. Residual skeleton structure cross pores are clearly observed in the scanning electron microscope images, which is established as a key factor in improving the properties of specimens. With the pretreatment of silica sol on sawdust, mechanical properties are enhanced and thermal conductivities could be optimized. Under the firing temperature of 1350 °C, the compressive strength of specimens with sawdust pretreated in 5 wt % concentration of silica sol is 2.4 MPa.