Abstract: Quaternized poly(dimethylaminoethyl methacrylate) (PQDMA) was synthesized via reversible addition-fragmentation chain transfer (RAFT) polymerization and alkylation reactions. Its antibacterial activity, biofilm clearance capacity, hemocompatibility, and cytocompatibility were systematically evaluated. The results revealed that PQDMA, a quaternary ammonium cationic polymer, exhibited antibacterial selectivity distinct from the cationic antibiotic gentamicin. The minimum bactericidal concentration (ρ_MB) of PQDMA against methicillin-resistant Staphylococcus aureus (MRSA) was reduced to 16 μg/mL, a quarter of that against wild-type S. aureus (64 μg/mL). PQDMA also showed stronger membrane depolarization effects and superior permeability against MRSA biofilms.This paradoxical selectivity may stem from MRSA’s membrane structural remodeling during resistance development, which potentially enhances the binding affinity between the cationic polymer and the bacterial membrane. Furthermore, PQDMA demonstrated excellent biocompatibility: its minimum concentration causing 10% hemolysis (HC₁₀) exceeded 1024 μg/mL, and its half-maximal inhibitory concentration (IC₅₀) was 128 μg/mL, indicating favorable hemocompatibility and cytocompatibility within its effective bactericidal range. This study offers novel insights for developing cationic polymer materials with structural targeting for antibiotic-resistant bacteria .