Fe3O4 magnetic nanoparticles were first prepared by chemical coprecipitation method, and then SiO2 was coated on Fe3O4 nanoparticles by hydrolyzation of tetraethyl orthosilicate. Subsequently, amino-functionalized magnetic nanoparticles (A-MNPs) were obtained by grafting KH-550 on the Fe3O4@SiO2. The microscopic morphology, chemical composition and magnetic properties of nanoparticles were tested by TEM, FTIR and VSM. Next, A-MNPs were applied to purify oily wastewater. It was found that surface amino-functionalization of A-MNPs can improve their pH-sensitivity, thus, the reuse of A-MNPs can be realized by changing pH value. When the pH value of solution was 4, A-MNPs exhibited the best oil removal effect. When the concentration of A-MNPs was 200 mg/L, the light transmittance of the simulated oily wastewater was 99.4%. However, when the concentration of A-MNPs was 500 mg/L, the transmittance could be kept above 80% even after 10 times of repeated use. In addition, A-MNPs had better purification effects for various simulated sewage composed of different concentrations of sodium chloride, kaolinite, emulsified oil and partially hydrolyzed polyacrylamide (HPAM). 375 mg/L A-MNPs could make the light transmittance of complex sewage reach more than 86.5% after purification.