The invention discloses a preparation method of N-doped carbon quantum dots, and a product and application thereof. The method comprises the following steps: dissolving chitosan in dionized water, placing in a hydrothermal reaction kettle at certain temperature to react, after a synthetic product is naturally cooled, separating, purifying, freeze drying and obtaining powder of the N-doped carbon quantum dots. Compared with other preparation method of the carbon quantum dots, the preparation method of the invention is simple in operation, only needs on raw material, needs no other surface passivating agent and is high in reaction speed, the prepared carbon quantum dots have high fluorescence and good biological compatibility, and a yield of the carbon dots reaches 45 percent. The carbon quantum dots not only can promote the growth of plants, but also can mark the plants. The application prospect in the fields of biology and agriculture is wide in the future.