Using the dispersion controlled feature of chalcogenide photonic crystal fiber, the broadband ultralow dispersionflattened chalcogenide photonic crystal fibers was designed, and the multipole method was used to research the effects of the hole pitch and the aperture ratio on the dispersion curves. By optimizing the diameter of the air holes of different layers in cladding, the structure with 0.7 μm air hole radius of inside two layers, 0.8 μm air hole radius of outer two layers and 5 μm hole pitch were obtained. The simulation results show that this structure is broadband flattened dispersion in 3~5 μm, and the absolute value of dispersion is less than 3.8 ps·nm－1·km－1.