The preparation method comprises the following steps: uniformly mixing the following components in percentage by weight: 0.05 to 0.1 percent of lanthanum chloride, 0.3 to 2 percent of chitosan, 0.1 to 0.2 percent of methylparaben, 0.5 to 1 percent of fruit juice extracting solution, 0.5 to 5 percent of xylitol, 0.5 percent of PEG-40 hydrogenated castor oil, 10 to 20 percent of glycerol and the balance of deionized water which is added to 100 percent by weight through a stirring device to obtain a mouthwash solution; the components are fully dissolved; and 2, heating the solution to 50-65 DEG C, and cooling to obtain the required mouthwash. The lanthanum chloride has the effects of mineralizing teeth and preventing decayed teeth, the chitosan has the effects of resisting inflammation and promoting tissue repair, the methylparaben has the preservative effect, the fruit juice extracting solution is used for seasoning, such as grape flavor or strawberry flavor, and the xylitol has the seasoning effect, is a sweetening agent, and has the effects of diminishing inflammation, resisting bacteria and increasing the cool feeling of the mouthwash; the PEG-40 hydrogenated castor oil has the effects of an emulsifier, a solubilizer and a stabilizer, and the glycerol has the effects of a solvent and an adhesive, so that the smooth feeling is improved, and the mouth wash has light sweetness.