The present invention discloses a stevia rebaudiana and oat germ enzyme and black fungus beverage and a preparation method thereof. Each 1000 g of the beverage comprises an enzyme raw liquid by fermenting 0.72-0.84 g of the stevia rebaudiana and using the oat germ as a carbon source, black fungus broken grains manufactured by 20-35 g of the black fungus, and the balance being RO water. The stevia rebaudiana and the black fungus are used as raw materials, the raw materials are subjected to a microorganism fermentation, the nutritional essence of the stevia rebaudiana is preserved, and at the same time, the nutritional benefits of the stevia rebaudiana are fully played. The beverage is rich in a variety of active enzymes, vitamins, minerals and microbial metabolism products needed by human body, good in mouthfeel, and low in sugar content, has functions of lowering blood sugar and lipids, and preventing thrombosis, tumors and oxidation and other important physiological functions, and is suitable for a long-term drinking and brings a healthy and innovative product to consumers. The preparation method is mild in reaction, low in production cost, simple and easily controllable in operation processes, and easy to implement large-scale industrial production.