In this study, a polymeric magnetic emulsion-breaking compound of ferric tetroxide and cationic polyacrylamide (Fe3O4@CPAM) was designed and synthesized, and its functional groups, chemical bonding, magnetic properties, and thermal stability were characterized. Finally, the pH-responsive behavior of the microwave-modified magnetic nanoparticles (MNPs) in terms of their demulsification effect on thick oil-in-water emulsions was investigated using the morphology and distribution of oil droplets, zeta potential, and contact angle of the MNPs. The results showed that with increasing pH, the water separation of the emulsion initially decreased and then increased, and the best emulsion-breaking effect was achieved at pH 3. The highest water partition of the emulsion was 64.39% at a concentration of 175 mg/L for Fe3O4 MNPs alone before modification. Under the same conditions, the water separation rate of the emulsion for Fe3O4@CPAM was 78.95%, indicating that, compared with the conventional demulsification method, microwaves can promote oil-water separation. The highest water separation rate of MNPs before microwave modification was 85% at pH 3, whereas the water separation rate of MNPs after microwave modification reached 94.70% under the same conditions. This proves that microwaves and modified MNPs have a synergistic emulsion-breaking effect, mainly because the combination of microwaves and polymer modification enhances the hydrophobicity and neutralizes the charge of the MNPs, thus improving the emulsion-breaking efficiency.