Compared with silica glass, chalcogenide glass possesses some unique advantages, such as high refractive index (2.0～3.5), low phonon energy (lower than 350 cm－1), and large infrared transmission window (up to 25 μm). In this paper, a kind of environmental protected Ge20Sb15Se65 chalcogenide glass was prepared, which showed good transmission in IR range. Based on this glass, a photonic crystal fiber with three rings air holes was designed. Multipole method was employed to study the dispersion properties of the designed photonic crystal fiber. The relationship between fiber structure parameters（the air diameter d, period Λ and the ratio d/Λ）and dispersion properties was presented. The simulation results showed that the zero dispersion point of photonic crystal fiber can be flexibility tuned to short or long wavelength region by control the fiber structure parameters. Finally, by optimizing the sturcure parameters, the dispersion flat photonic crystal fiber through of 3~5 μm was obtained, with the dispersion value less than 5 ps·nm－1·km－1.