The development of efficient, fast-acting demulsifiers that function effectively at low temperatures is crucial for treating high-water-content crude oil fluids in sustainable oilfield development. In this study, a series of biodegradable and highly polar polyaspartamide-based demulsifiers were synthesized via aminolysis of polysuccinimide (PSI) using aliphatic and aromatic amines. Demulsification tests revealed that PSI modified with phenethylamine (PSI-PEA) exhibited no phase separation capability, even at high dosages and 60.0 °C. In contrast, PSI grafted with 3-dimethylaminopropylamine (PSI-DA) demonstrated excellent low-temperature efficiency, rapidly breaking oil-in-water (O/W) emulsions containing 1.0−10.0 wt% oil. More notably, the co-grafted derivative PSI-PEA-DA showed outstanding versatility and rapid action, achieving up to 99.9% demulsification efficiency for O/W emulsions across a broad pH range (4.0−9.0) and oil content (1.0−20.0 wt%) under ambient conditions within 10 min. Dispersion behavior studies indicated that the aromatic side-chain-modified PSI-PEA tends to form hydrophilic micelles in the aqueous phase, thereby losing its interfacial activity and demulsification capability. Replacement asphaltene experiments, noncovalent interaction (NCI) analysis, and dispersion state microscopy of the co-grafted PSI-PEA-DA demonstrated that the strategic incorporation of aromatic side chains significantly enhances interactions between the demulsifier and natural surfactants. Density functional theory (DFT) calculations further demonstrated that the polyamide backbone of these demulsifiers exhibits stronger polarity than conventional oxygen-based polymers, facilitating stronger binding with the natural surfactants constituting the oil–water interfacial films. Therefore, enhancing polarity and aromaticity of the demulsifier while preserving its structural characteristics represents an effective strategy to reduce the demulsification temperature and improve both the efficiency and speed of demulsification.