The invention discloses a rosa roxburghii polysaccharide, as well as a preparation method and application thereof. The preparation method mainly comprises raw material pretreatment, extraction, deproteinization, de-coloration, alcohol precipitation, as well as separation and purification, wherein the extraction is to mix pretreated rosa roxburghii dry powder and water in a mass volume ratio of (1to 15) to (1 to 35), and perform extraction at a temperature of 55 to 95 DEG C; perform centrifugal separation to obtain a rosa roxburghii polysaccharide extracting solution, and concentrate the rosaroxburghii polysaccharide extracting solution under reduced pressure to 1 / 3 to 1 / 6 of the original volume to obtain a concentrated solution of the rosa roxburghii polysaccharide. The rosa roxburghii polysaccharide prepared by the invention has the advantages of high purity and uniform molecular weight. Compared with the existing rosa roxburghii polysaccharide, as proved by in-vitro fermentation experiments, the rosa roxburghii polysaccharide disclosed by the invention can promote the generation of short-chain fatty acids such as acetic acid, propionic acid and butyric acid, promote the growthof beneficial bacteria such as lactic acid bacteria and bifidobacterium, and can be taken as prebiotics. Meanwhile, the rosa roxburghii polysaccharide can inhibit the activity of alpha-glucosidase, and can be applied to healthy foods or medicines having auxiliary therapy effects on intestinal diseases and diabetes mellitus.