The separation of oil-water mixtures in highly acidic, alkaline, and salty environment remains a great challenge. Simple, low-cost, efficient, eco-friendly, and easily scale-up processes for the fabrication of novel materials to effective oil-water separation in highly acidic, alkaline, and salty environment, are urgently desired. Here, a facile approach is reported for the fabrication of stable hydrogel-coated filter paper which not only can separate oil-water mixture in highly acidic, alkaline, and salty environment, but also separate surfactant-stabilized emulsion. The hydrogel-coated filter paper is fabricated by smartly crosslinking filter paper with hydrophilic polyvinyl alcohol through a simple aldol condensation reaction with glutaraldehyde as a crosslinker. The resultant multiple crosslinked networks enable the hydrogel-coated filter paper to tolerate high acid, alkali, and salt up to 8 m H2SO4, 10 m NaOH, and saturated NaCl. It is shown that the hydrogel-coated filter paper can separate oil-water mixtures in highly acidic, alkaline, and salty environment and oil-in-water emulsion environment, with high separation efficiency (>99%).