NOVELTY - The present invention relates to a composite material windmill blade structure and a fabrication method, in which the light-weight and high strength characteristics of composite materials are utilized, wherein a preformed blade structure includes three parts: a preformed metal adapter, which is an adapter structure capable of externally connecting with a metal rotary shaft and internally connecting with long fiber layers; a preformed styrofoam liner (foaming grade polystyrene) for supporting the entire structure and generating the in-mold uniform pressing and elastic effect; and a preformed dry long fiber layers serving as a structure form for encapsulating and reinforcing the exterior of the preformed styrofoam liner. The forming method employs a resin transfer molding method to inject epoxy resin into a configured mold, such that the preformed long fiber layers, metal adapter, and styrofoam liner may be fully glued and immersed with the epoxy resin in a mold cavity and be integrally thermal curing formed under the set temperature, so as to reduce the structure weight and manufacturing cost, and further employs the high strength structure to enhance the overall blade supporting effect as the primary object. The secondary object of the present invention is to fabricate the styrofoam blade by the steam foaming and glue forming method, which subjects the original polystyrene material for styrofoam to high temperature steam to be foamed as a semi-finished product of granular styrofoam balls, and then places the granular styrofoam balls into the blade mode for high temperature second foaming and gluing process. The other secondary object of the present invention is to provide a preformed long fiber layers structure, which employs the styrofoam liner and a setting agent to thermally press the dry long fiber layers as a transient preformed blade structure by pressing forming method, in which the long fiber layers are formed by stacking and gluing layer by layer with the fabric having the warp and weft crossly arranged or the fiber cloth in single direction combining with the setting agent.