Polyelectrolyte　complex　nanoparticles　(PEC　NPs)　armed　with　tunable　quaternary　ammonium　groups　are　designed　from　dextran　sulfate　sodium　and　chitosan　(CS),　and　utilized　as　novel　building　blocks　to　prepare　quaternized　polyelectrolyte　complex　membranes　(QPECMs)　for　nanofiltration　process　via　surface　coating　and　glutaraldehyde　crosslinking　method.　Structural　characteristics,　surface　properties　and　separation　performance　of　QPECMs　are　compared　with　these　of　quaternized　CS　membrane　(QCSM)　and　nascent　PEC　NPs　membrane　(PECM).　Results　indicate　that　increased　water　permeability　of　QPECMs　generates　from　loose　aggregation　architecture　moderating　tight　packing　of　polymeric　chains,　while　enhanced　ion　selectivity　is　attributed　to　efficient　charge　character　providing　strong　electrostatic　repulsion　toward　metal　cations.　Moreover,　QPECM-4　features　efficient　anti-scaling　properties　with　low　flux　decline　ratio　of　11.5%　and　high　recovery　ratio　of　96.5%,　accompanied　with　long-term　stability　and　recyclability　in　gypsum　dynamic　filtration.　This　design　strategy　of　water-soluble　PEC　NPs　serving　as　building　blocks　presents　facile　adaptability　to　other　polyelectrolyte　complex　membrane　systems　for　specific　water　treatment　and　environmental　remediation.