The invention discloses water-soluble quantum dots and a preparation method thereof. The preparation method comprises the following steps: dropwisely adding oil-soluble quantum dots into a polymer stock solution, and after finishing dropwise addition, stirring at room temperature for 2.5-3.5 hours, thereby obtaining polymer-interpenetrating oil-soluble quantum dots; and adding an organic crosslinking molecule stock solution into the polymer-interpenetrating oil-soluble quantum dots, stirring for 25-35 minutes after finishing dropwise addition, repeatedly dissolving and precipitating the product, and filtering for purification, thereby obtaining the water-soluble quantum dots. The polymer interpenetrates into the oil-soluble quantum dots, and the organic crosslinking molecules are added into polymer-interpenetrating oil-soluble quantum dots, thereby obtaining the quantum dots with hydrophilic effect. The water-soluble quantum dots prepared by the method have favorable fluorescence intensity and stability, and the quantum dot confinement effect of the quantum dots after phase transition is not influenced.