The invention discloses a graphene modified composite glass fiber and a preparation method thereof. The preparation method comprises the following steps: (1) preparing glass fiber; (2) pretreating the glass fiber; (3) weighing graphene quantum dots, and preparing dispersion solution with the concentration of 0.1-1mg/ml, wherein a solvent is water, acetone or dimethyl sulfoxide; carrying out ultrasonic stirring on 80-100ml graphene quantum dot dispersion solution, adding the pretreated glass fiber into the dispersion solution, halving ultrasonic stirring rate, and dipping for 30-60 minutes; and taking out the glass fiber, cleaning, and drying for later use; and (4) preparing the graphene modified composite glass fiber. The obtained composite glass fiber has excellent mechanical strength, raw materials are available, industrial production can be easily realized, and application range of the glass fiber composite material is further expanded.