The invention relates to a preparation method of a ZnO / GQD-PEI composite quantum dot antibacterial agent. The preparation method comprises the following steps: heating citric acid to obtain an orangesolution; dropwise adding a NaOH solution to adjust pH, and performing dialysis and rotary evaporation, thus obtaining a graphene quantum dot (GQDs) solution; firstly diluting the obtained GQDs solution, then adding modifier to adjust the concentration, performing the ultrasonic treatment, adding EDC-HCl in two times, ultrasonically stirring, and obtaining GQD-PEI; separately adding an alkaline source and a zinc source into an ethanol solution, stirring, and obtaining a solution A and a solution B; adding the obtained GQD-PEI into the solution B, ultrasonically dispersing, then adding the solution A into the solution B, heating in a water bath, reacting, then washing, and centrifuging, thus obtaining the composite quantum dot antibacterial agent. Compared with the traditional antibacterialagent, the nano composite antibacterial agent prepared in the invention has the advantages of safety, environmental protection, high drug resistance and the like, and the minimum antibacterial concentration for escherichia coli can reach 1.8 mg / ml.