Attapulgite/poly(acrylic acid) nanocomposite (ATP/PAA) hydrogels with high clay content of around 90% have been synthesized by a novel surface-initiated redox radical solution polymerization of acrylic acid (AA) with the 3-aminopropyltriethoxysilane (APTES) modified attapulgite nanorods (ATP-NH2) as initiator and the 3-methacryloxypropyltrimethoxysilane (MPTMS) modified attapulgite nanorods (ATP-C=C) as cross-linker. High inorganic-content nanocomposite (ATP/PAA) hydrogel with a three-dimensional network structure was obtained via the linkage of the different functional attapulgite nanorods with the grafted poly(acrylic acid) (PAA) chains. Both the two kinds of the functional attapulgite nanorods acted as the cross-linking sites in the nanocomposite hydrogel. Due to its unique structure, a remarkably high adsorption capacity of the cationic dye (methylene blue (MB)) of 308.0 mg/g was achieved when the ATP/PAA nanocomposite hydrogel was used as the adsorbent for water treatment.