As a precondition , it was presented that the single_mode optical fiber , the dispersion_shift optical fiber and the dispersion_flatten optical fiber can be regarded as the special examples of the index_guiding photonic crystal fiber(PCF) under a determinate condition, when the new step effective index model （SEIM） was used to discuss the chromatic dispersion properties of PCF. The result compared with the experimental data reported in the literature indicated that such a SEIM can be used to uncover more interesting dispersion regimes in PCFs. In particular, the speed of calculation by using the SEIM is very quick. Finally, the dispersion characteristics of the PCF with different air hole diameter d, air hole pitch Λ and the core radius a were also discussed.