The invention discloses fermented mango juice and a preparation method thereof. The preparation method comprises the following steps: S1: uniformly mixing a mango raw material juice, reducing sugar and water, and performing filtering and sterilization, so as to obtain a mixed solution; S2: inoculating a leavening agent when the mixed solution is cooled to a preset temperature, and performing fermentation at the preset temperature, so as to obtain the fermented mango juice; S3: performing homogeneous refinement on the fermented mango juice. The reducing sugar comprises, but not limited to, oneor more of glucose, galactose, lactose and fructose corn syrup, and the mixed solution contains 0.5 to 2 mass percent of the reducing sugar; the leavening agent comprises, but not limited to, probiotics; the adding amount of the probiotics is 1 x 10<6> to 1 x 10<9> cfu/mL, and the fermentation time is 12 to 96 hours. The fermented mango juice does not contain a stabilizer, is stable in system, tastes refreshing, sour, sweet and delicious, low in slurry viscosity and high in flowability, can be directly applied to other products or used as an ingredient applied to other products. Furthermore, the preparation method of the fermented mango juice is simple in process and suitable for industrial large-scale production.