Focusing on hydrophilic surface modification of poly(methyl methacrylate)/poly(methyl methacrylate-co-acrylic acid) (PMMA/PMMA-co-PAA) composite film, a copolymer is synthesized to improve the hydrophilicity and compatibility with PMMA matrix, and the composite film is subjected to surface activation using a sodium hydroxide solution. The infrared absorption peaks corresponding to –OH and –C═O functional groups become progressively weaker with the increase of copolymer content, indicating the formation of intermolecular hydrogen bonds. The hydroxyl absorption peak is significant and two new absorption peaks corresponding to the symmetric contraction and antisymmetric contraction of –COO– appear, indicating that sodium hydroxide solution can disrupt the intermolecular hydrogen bond and react with acrylic acid to form sodium acrylate to make the surface contain sodium elements. The elemental sodium content on the composite film can reach 1.9% and the nanostructures contain 20 times more sodium than the smooth parts, indicating the sodium acrylate can induce surface segregation of PMMA and form nanoparticle-like structures to improve surface roughness. The hydrophilic component combined with the surface structure makes the contact angle less than 10°. Furthermore, thermostability can be slightly promoted by introducing the copolymer which is closely related to the intermolecular hydrogen bonding formed directly between the copolymer and the PMMA matrix.