The invention aims at providing fishbone fermented milk and a preparation method thereof. Fishbones are subjected to enzymolysis through protease, then hydrolysate and milk are mixed, and lactic acid bacteria are introduced for fermentation so that the fishbone fermented milk can be prepared. The fishbone fermented milk is pure white or light yellow in luster, has the special fresh aroma of aquatic products and milk and is free of odors making people unpleasant; the saline taste, the sweet taste and the delicate flavor are palatable, and the requirements of the popular taste are met; the fishbone fermented milk is uniform and free of suspended solids and flocculent precipitate and contains functional peptide, vitamins, mineral substances, trace elements and other rich nutrient substances. According to another large characteristic of the fishbone fermented milk, the fishbone fermented milk has the calcium supplement function. Close relationship exists among absorption and utilization of calcium, the form of intake calcium, a proper calcium and phosphate ratio and existence of amino acid. Fishbones contain rich calcium and phosphate and other substances, fishbone calcium is converted into precipitate calcium, amio acid calcium, calcium lactate and the types of various kinds of organic calcium, the concentration of soluble calcium and the acidity of bone liquid are increased, calcium is prevented from being precipitated in the intestinal tract, and absorption of a human body to calcium is promoted.