A novel method, which involves the coating of the inactive Al2O3 microspheres with silica sol, is employed to decrease the sintering temperature for preparing porous alumina ceramics, and the influences of coating thickness and sintering conditions are discussed. The results show that the mullite necks among packed Al2O3 spheres are formed with the help of colloidal coatings at 1550 A degrees C, and the uniform and smooth pores are well reserved without visible shrinkage. In addition, the diffusion-controlled nucleation-growth process of mullite neck is set up based on EDS and XRD analysis, and the influence of inert surface of Al2O3 microspheres on mullitization kinetics is also revealed. Furthermore, the sintered necks grow and strengthen with the increase of the coating thickness because the available melt for thicker coating is more sufficient to meet capillary force. Besides, the ceramic strengthens with the increase of mullite content in necks, and the fracture mechanism transforms from glass crack to grain-boundary crack when the composition of necks changes from mullite/glass compounds to mullite. Finally, the synthesized porous ceramic shows excellent permeation flux, and its favorable pollution resistance is proved.