The invention discloses a medical hemostatic sponge and a preparation method. The sponge is a milk-white loose and porous sponge material composed of bletilla mannan extracted from the dry tubers of orchid bletilla, wherein the essential of the preparation method is that mannan in bletilla is extracted out by water extraction and alcohol precipitation processes at first, and then is prepared into the loose and porous sponge material by a vacuum freeze-drying technology. The sponge is wide in material source, low in cost, non-immunogenicity, high in adhesion speed for wound surface, high in adhesion firmness, remarkable in physical hemostatic effect, soft in performance, capable of being used without pre-treatment, extremely high in cost performance, and capable of replacing the existing gelatine hemostatic sponge.