The invention provides a method for preparing blue emission carbon quantum dots. According to the method, carbon quantum dots are synthesized by a microwave method, and dehydration-condensation reaction generated under the microwave effect between amino acid and ethylene glycol is utilized to form carbon quantum dots. The method is a one-step preparation method, is quick, efficient and simple. Furthermore, the invention further develops application of the blue emission carbon quantum dots in oneness detection of antibiotics, and provides a novel method for quantitatively detecting metronidazole. The aim of quantitative detection can be achieved by utilizing optical sensing between metronidazole and carbon quantum dots, and the method is quicker, simpler, more convenient and more intuitive than other methods.