In this paper, polyacrylic acid (PAA)/TiO2 nanocomposites with photocatalytic activity were prepared via in-situ growth of anatase TiO2 nanoparticles in polyacrylic acid matrix using a simple hydrothermal method. First, water-soluble titania xerogel was prepared by a modified sol-gel procedure involving hydrolyzation of titania precursor, tetra-n-butyl titanate (TBT), in trifluoroacetic acid solution. Then, titania xerogel was dissolved in water and formed a homogeneous solution. Sequentely, polyacrylic acid as an adsorbed polymer was put into the solution and a hydrogel containing titania xerogel was obtained. After hydrothermal treatment for 10h at 100 °C, polyacrylic acid/TiO2 nanocomposites were obtained. The structure and morphology of as-prepared samples were characterized by Raman, SEM, and TEM. The photocatalytic activity of nanocomposites was evaluated by photodegradation of methyl orange under UV irradiation. The result showed that TiO2 nanocomparticles in anatase phase was in-situ generated inside PAA matrix and revealed a high photocatalytic activity, which can be used in environment purification and water treatment.