The equivalent-refractive index model is implemented based on the fundamental space filling mode (FSM), the photonic crystal cladding is considered as a uniform medium, then the photonic crystal fiber is considered as a step-index fiber. The mode characteristics of the photonic crystal fibers (PCF) are investigated in detail using the equivalent-refractive index model, including the propagation constant, the mode-field distribution, the power confinement factor, power loss and dispersion. With the PCF and SMF compared, it is shown that the PCF is almost an endlessly single-mode fiber. The PCF can be used to improve the pump efficiency of EDFAs because its power confinement factor is higher than that of the conventional erbium-doped fibers.