The invention discloses a composition for reducing bone loss and a preparation method thereof. The composition is prepared from a calcium compound, marine fishbone collagen oligopeptide powder, glucosamine hydrochloride, casein phosphopeptides, natural vitamin E, vitamin C, vitamin K1, vitamin B12, vitamin D3 and dietary fiber. As proved by animal and human body tests, the composition has the effects of increasing bone density, enhancing bone intensity, reducing bone loss, delaying aging and enhancing the immunity, and can be taken by the middle-aged and old as a health-care food for a long time. The preparation method is scientific and reasonable, and is suitable for industrial production.