The invention discloses a hollow floor construction method using an opening composite core mold. The composite core mold consists of a latticed framework, a face layer and at least three foot supporters, wherein the face layer and the framework jointly define a geometric body; the geometric body is hollow; an opening is formed in the bottom of the geometric body; and the foot supporters are positioned at the lower side of the opening and are exposed out of the face layer. The method has the critical steps that after the opening composite core mold is put in a gap position between ribs according to design requirements, concrete is cast for the first time to reach the opening position of the opening composite core mold, and vibration maintenance is performed; and then, the concrete is cast for the second time until the whole of the latticed ribs and cavity components is covered, and then, maintenance and mould removal are performed to form a cast-in-place concrete hollow floor. Compared with a conventional cavity construction process with the defects of complicated process and great construction difficulty, the hollow floor construction method provided by the invention has the advantages that the construction period can be effectively shortened; the construction difficulty is reduced; the labor cost is greatly reduced; and the transportation and storage of products are convenient.