Nanofiltration (NF) technology has emerged as a promising water treatment technology owing to its low energy consumption and high water flux. However, conventional negatively charged NF membranes are incapable of satisfying the demands of hard water softening, and severe membrane fouling still acts as a critical bottleneck limiting their practical application. Herein, a novel dual-polymer cross-linking strategy is developed to construct positively charged sieving layers on ultrafiltration substrates through a one-step coating and thermal curing approach, in which zwitterionic polyethyleneimine (QSPEI) and polyacrolein (PA) serve as dual building blocks. The interpenetrating covalent cross-linking network between QSPEI and PA macromolecules forms a compact and stable separation layer, enabling the successful fabrication of high-performance composite NF membranes. Under optimal preparation conditions, the obtained membrane exhibits a high MgCl₂ rejection of 95.1% and an excellent pure water permeance of 10.9 L·m⁻²·h⁻¹·bar⁻¹, while maintaining a high flux recovery rate of 91.0% against 100 ppm BSA fouling. The prominent antifouling performance, comparable to that of typical zwitterionic NF membranes, originates from the coexisting positive and negative charges on QSPEI chains induced by quaternization and sulfonation modification. Meanwhile, the dense cross-linked network endows the membrane with outstanding separation efficiency toward both monovalent and divalent cations. This work provides a feasible fabrication strategy for advanced NF membranes with synergistic separation and antifouling performance, possessing great application potential in water softening and ion separation.