The present invention discloses a method for removing impurities from a porous material by flowing a supercritical fluid with or without a modifier therein over a to-be-cleaned porous material having pores at the nanometer level under suitable temperatures and pressures, so that the supercritical fluid migrates into the pores at the nanometer level and removes impurities entrapped therein. The method of the present invention uses the physical and chemical properties of the supercritical fluid and optionally the modifier to clean the porous material without using an acid or alkaline solvent. The method of the present invention conserves water, and is both a highly efficient and environmentally friendly cleaning technique.