The invention discloses a preparation method of a pure-brewed fermented Noni product. The method is characterized by comprising the following steps: taking Noni as a raw material, picking ripe Noni, selecting better Noni, selecting best Noni, and conducting washing, drying, baking and pulping; conducting squeezing, filtration, purification and extraction; and conducting sterilization, filling, sealing, bottle reversing, inspection, spraying, labeling, packaging, inspection for qualifying, finished product acquirement and third-party inspection. The method adopts leuconostoc mesenteroides to carry out fermentation on crushed Noni pulp fluid, not only releases the activity and the nutrient components of the Noni more effectively, improves the utilization rate of the Noni, but also increasesthe health-care functions of fermentation liquor. At the same time, the method solves the problem that Noni is difficult to ferment in the prior art, and the product is suitable for a wide range of people.