The utility model discloses a concrete core slab prefabricating inner die and an assembling frame, wherein the prefabricating efficiency of a concrete core slab is improved. The prefabricating inner die comprises steel pipes, a supporting frame and outer wrapping cloth. The steel pipes are placed on the periphery of the supporting frame. The corners of the supporting frame and the steel pipes are tied up and fixed. The outer wrapping cloth wraps the steel pipes and is tied up and fixed. When the assembling frames are used, the number of the assembling frames and the lengths of the steel pipes are determined according to the length of the concrete core slab, and the concrete core slab prefabricating inner die is manufactured in a segmented mode. When the inner die is spliced through the steel pipes, operation is convenient, replacement and maintenance are easy, material selection is convenient, and manufacturing cost is low. The concrete core slab prefabricating inner die is easy and rapid to assemble and is especially convenient and rapid to detach, and personnel do not need to enter the inner die to conduct operation. Labor is saved, the work progress is accelerated, and safe production is ensured. The concrete core slab prefabricating efficiency is remarkably improved, and production cost is reduced.