The utility model discloses a combined ladder-shaped trapezoidal thread, comprising a front guide part (1), a rough cutting part (2), a fine cutting part (3), a back guide part (5) and a clamping part (6), wherein the front guide part (1), the rough cutting part (2), the fine cutting part (3), the back guide part (5) and the clamping part (6) are orderly connected; the front ends of the rough cutting part (2) and the fine cutting part (3) are both in taper-like shape and are equipped with a circular exhaust groove (4); the rough cutting part (2) and the fine cutting part (3) are same in the cutter tooth pitch and are consistently perforated. The combined ladder-shaped trapezoidal thread provided by the utility model has the advantages that the operation of rough and fine cutting is completed in one time by means of section cutting, the effect for processing the ladder-shaped internal thread of the material with large pitch or difficult process is good, the accuracy of the ladder-shaped internal thread processed can be stably reached to 5 H to 6 H; furthermore, the processing cycle is shortened, the production cost is reduced, the tool can be installed and replaced simple and quickly, and the surface fineness of the internal thread is improved.