The paper proposes octagonal structural photonic crystal fiber (PCF) with double cladding and simulated numerically its dispersion properties by the multipole method. The result shows that the dispersion properties of the octagonal structural photonic crystal fiber have large differentiation by changing Λ or d1. But they change slowly by changing d2,while they are dependent on the value of d2. When d2 is small,the dispersion properties change with d2,but when it is larger than twice of d1,the dispersion properties will no longer change. According to this characteristic,a nearly zero dispersion flattened photonic crystal fiber at 1500 nm is designed. The result has good effect on preparing the doping photonic crystal fiber which is used in the high power laser and improving the laser output quality.