The invention discloses a concrete core slab prefabricated internal mold, an assembling frame and an assembling method for improving the prefabricating efficiency of the concrete core slabs. The prefabricated internal mold comprises a steel pipe, a support frame and outer wrapping cloth, wherein the steel pipe is arranged at the periphery of the support frame, the corners of the support frame are tied up with the steel pipe in a fixed manner, and the outer wrapping cloth is wrapped outside the steel pipe and is tied up and fixed; when the assembling frame is used, the quantity of the assembling frame and the length of the steel pipe are determined according to the length of the concrete core slab, and the concrete core slab prefabricated internal mold are manufactured segmentally; the steel pipe splicing operation is convenient, the steel pipe is easy to replace and maintain, the material selection is convenient and the manufacture cost is low; the concrete core slab prefabricated internal mold can be assembled simply and rapidly, especially the internal mold can be conveniently and rapidly disassembled without needing operators to enter the internal mold to operate, therefore, the labor force is saved, the project progress is accelerated, the production safety is ensured, the concrete core slab prefabricating efficiency is remarkably improved and the production cost is reduced.