The invention relates to antibacterial crosslinked nanoparticles and a preparation method thereof. The method comprises the steps of firstly, adopting solution polymerization to synthesize poly [2-(N,N-dimethyl amino)ethyl methacrylate] (PDMAEMA) under the regulation of a reversible addition fragmentation chain transfer (RAFT) agent-CPND; then, under the acidic condition, taking the PDMAEMA as a macromolecule RAFT agent, and adding oil-soluble monomer methyl methacrylate (MMA) and a crosslinking agent-ethylene glycol dimethacrylate (EGDMA) in a semi-continuous charging way to prepare crosslinked nano emulsion for soap-free antibacterial particles. After that, the nanoparticles are enabled to be subjected to quaternization, and the good antibacterial property of the nanoparticles is verified by taking gram-negative bacteria escherichia coli as an experimental strain and using a spread-plate method.