In this paper, a facile approach was developed for highly effective oil/water separation by incorporating of the dimethyldiallylammonium chloride acrylamide polymer (P(AM-DMDAAC)) into graphene aerogels. The functionalized 3D graphene aerogel integrated a series of excellent physical properties, including low density (11.4 mg/cm3), large specific surface area (206.591 m2/g), and great hydrophobicity (contact angle of 142.7°). The modified aerogel showed excellent adsorption capacity for oils and organic solvents (up to 130 g/g). The saturation can be reached in a short time and the adsorption capacity remained nearly unchanged after repeated heating cycles. Meanwhile, we found a simple method to achieve controlled wettability transition of P(AM-DMDAAC)/graphene aerogels (PGAs) by changing the pH values. The hydrophobic PGA prepared at pH 2.03 showed outstanding oil/water separation performance (130 g/g). As the pH increased, the oil adsorption capabilities of PGAs decreased slightly, but the adsorption performance for the hydrophilic organic dye was significantly improved. Therefore, as a recyclable and efficient water purification material, the sustainable and environment-friendly polymer-modified graphene aerogel has great application potential.