The invention relates to preparation methods of a natural biomass quantum dot and a biomass quantum dot-copper nanocomposite and application of the natural biomass quantum dot. The preparation methodof the biomass quantum dot comprises the following steps: S1, grinding spinach leaves into paste; S2, adding a mixed solution of ethanol and acetone, uniformly mixing and filtering; S3, adding oleic acid, polyethylene glycol diamine and a magnetic stir bar into a three-neck flask, pumping air out after sealing, introducing argon, stirring, heating to 170 to 190 DEG C, and stopping heating when thesolution is in orange-yellow color; S4, cooling the solution to room temperature, adding a chlorophyll crude extracting solution, heating the solution to 170 to 190 DEG C under the protection of theargon and magnetic stirring, and keeping the temperature for 170 to 190 minutes; S5, naturally cooling the solution to room temperature, and adding concentrated HCl till the solution is in strong acidity; S6, standing for layering, and collecting subnatant; S7, adjusting the pH value of the solution to neutral with a saturated NaOH solution, filtering larger particles, and performing dialysis to obtain a biomass quantum dot solution. The biomass quantum dot solution has the characteristics of high fluorescence intensity, high light stability and high biocompatibility.