Rare earth-doped photonic crystal fiber (PCF) is a kind of ideal material to produce high power fiber lasers and amplifiers, which is particularly important to the research on rare earth-doped PCF. A new octagonal PCF with air hole and arranged in double rows is proposed. And the characteristics are analyzed. Influences from the changes of doped fiber coil diameters, doped refractive index and wavelength on effective model field areas and refractive index are researched. And overlap factor characteristics of the rare earth-doped fiber as a fiber amplifier are calculated. Theory bases of producing photonic apparatuses such as fiber amplifiers are established.