The invention relates to a preparation method of a carbon quantum dot fluorescent probe for detecting paraquat. The preparation method comprises the following two steps of firstly, accurately weighing0.03g of m-phenylene diamine, putting into anhydrous ethanol, dissolving, transferring into a hydrothermal reaction kettle with a polytetrafluoroethylene inner container, sealing, reacting for 12h ina constant-temperature drying box of which the temperature is increased to 160 DEG C by programs, naturally cooling to room temperature, fetching out, and putting the product into a beaker; adding anextender into the beaker, separating by a chromatography method, dialyzing the product by a dialysis bag, and diluting the remaining liquid by deionized water, so as to obtain the carbon quantum dotfluorescent probe with surface containing hydroxyl groups and carboxyl groups. The carbon quantum dot fluorescent probe has the advantage that when the spaces of the carbon quantum dot fluorescent probe with surface negative charges and the target molecule paraquat with positive charges are mutually approached under the mutual electrostatic function of cations and anions, the fluorescence resonance energy transfer is generated, the fluorescence intensity of the carbon quantum dot fluorescent probe is reduced, and the selective identification and sensitivity detection on the target molecule paraquat are realized.