Abstract: In order to prepare a new type of safe, stable and efficient nano silver composite antibacterial agent, silver nanoparticles/chitosan (AgNPs/CS) was biosynthesized by one-step method using Chimonanthus praeco petal extract and chitosan as reducing agent and stabilizing agent, respectively. The optimal preparation procedure was determined by single factor experiment. The products were characterized by UV-vis absorption spectroscopy, TEM and X-ray diffraction, and their antibacterial activity, anti-drug resistance, stability as well as biological safety were comprehensively evaluated. The experimental results show that the AgNPs/CS has the characteristic absorption peak of AgNPs at 451 nm, and the evenly dispersed AgNPs are spherical with average diameter of 12.83 nm, and the crystals have a face-centered cubic structure. The AgNPs/CS show antibacterial activity against aquatic pathogens both in vitro and in vivo, and has excellent anti-drug resistance, biosafety and stability. Therefore, AgNPs/CS is an ideal composite antibacterial agent, which has a prospective application in aquaculture field.