The invention provides a preparation method of a transformer iron core with a foldable opening. The preparation method comprises the following steps of performing transformer iron core design, silicon steel band cutting, splitting and folding through a computer, and preparing into multiple silicon steel sheets with independent geometrical shapes, and combining into a framework iron core with the foldable opening or a triangular three-dimensional foldable type iron core group. The bending angles of the foldable iron core are 30 degrees, 45 degrees and 90 degrees; the four angles of the foldable iron core consist of eight 45-deree bending angles or twelve 30-degree bending angles; and the folding process has relatively low stress on the silicon steel sheets and does not need annealing treatment. By integrating the cutting and folding processes, the foldable iron core performs lamination for 1-3 sheets, so that a convenient space is reserved for assembling of a transformer coil, and the coil can be produced in advance and rapid assembling can be realized consequently.